Rewrite `setup.sh` to be POSIX-compatible

[bartekpacia]

$ shellcheck -s sh ./setup.sh | pbcopy

In ./setup.sh line 35:
    if [[ "$OS_NAME" == windows ]]; then
           ^-------------------------^ SC3010 (warning): In POSIX sh, [[ ]] is undefined.

In ./setup.sh line 51:
    if [[ "$archive_name" == *zip ]]; then
           ^---------------------------^ SC3010 (warning): In POSIX sh, [[ ]] is undefined.

In ./setup.sh line 91:
[[ -z $ARCH ]] && ARCH="$ARCH_NAME"
^------------^ SC3010 (warning): In POSIX sh, [[ ]] is undefined.

In ./setup.sh line 93:
ARR_CHANNEL=("${@:$OPTIND:1}")
            ^----------------^ SC3030 (warning): In POSIX sh, arrays are undefined.
              ^------------^ SC3057 (warning): In POSIX sh, string indexing is undefined.

In ./setup.sh line 94:
CHANNEL="${ARR_CHANNEL[0]}"
         ^---------------^ SC3054 (warning): In POSIX sh, array references are undefined.

In ./setup.sh line 96:
[[ -z $CHANNEL ]] && CHANNEL=stable
^---------------^ SC3010 (warning): In POSIX sh, [[ ]] is undefined.

In ./setup.sh line 97:
[[ -z $VERSION ]] && VERSION=any
^---------------^ SC3010 (warning): In POSIX sh, [[ ]] is undefined.

In ./setup.sh line 98:
[[ -z $ARCH ]] && ARCH=x64
^------------^ SC3010 (warning): In POSIX sh, [[ ]] is undefined.

In ./setup.sh line 99:
[[ -z $CACHE_PATH ]] && CACHE_PATH="$RUNNER_TEMP/flutter/:channel:-:version:-:arch:"
^------------------^ SC3010 (warning): In POSIX sh, [[ ]] is undefined.

In ./setup.sh line 100:
[[ -z $CACHE_KEY ]] && CACHE_KEY="flutter-:os:-:channel:-:version:-:arch:-:hash:"
^-----------------^ SC3010 (warning): In POSIX sh, [[ ]] is undefined.

In ./setup.sh line 101:
[[ -z $PUB_CACHE_KEY ]] && PUB_CACHE_KEY="flutter-pub-:os:-:channel:-:version:-:arch:-:hash:"
^---------------------^ SC3010 (warning): In POSIX sh, [[ ]] is undefined.

In ./setup.sh line 102:
[[ -z $PUB_CACHE_PATH ]] && PUB_CACHE_PATH="default"
^----------------------^ SC3010 (warning): In POSIX sh, [[ ]] is undefined.

In ./setup.sh line 113:
    elif [ "$OS_NAME" == "windows" ]; then
                          ^-- SC3014 (warning): In POSIX sh, == in place of = is undefined.

In ./setup.sh line 120:
if [[ "$TEST_MODE" == true ]]; then
   ^------------------------^ SC3010 (warning): In POSIX sh, [[ ]] is undefined.

In ./setup.sh line 121:
    RELEASE_MANIFEST=$(cat "$(dirname -- "${BASH_SOURCE[0]}")/test/$MANIFEST_JSON_PATH")
                                              ^---------------^ SC3028 (warning): In POSIX sh, BASH_SOURCE is undefined.
                                              ^---------------^ SC3054 (warning): In POSIX sh, array references are undefined.

In ./setup.sh line 126:
if [[ "$CHANNEL" == "master" || "$CHANNEL" == "main" ]]; then
   ^-- SC3010 (warning): In POSIX sh, [[ ]] is undefined.

In ./setup.sh line 132:
if [[ "$VERSION_MANIFEST" == *null* ]]; then
   ^-- SC3010 (warning): In POSIX sh, [[ ]] is undefined.

In ./setup.sh line 144:
    expanded_key="${1/:channel:/$version_channel}"
                      ^-----------------------------^ SC3060 (warning): In POSIX sh, string replacement is undefined.

In ./setup.sh line 145:
    expanded_key="${expanded_key/:version:/$version_version}"
                      ^-- SC3060 (warning): In POSIX sh, string replacement is undefined.

In ./setup.sh line 146:
    expanded_key="${expanded_key/:arch:/$version_arch}"
                      ^-- SC3060 (warning): In POSIX sh, string replacement is undefined.

In ./setup.sh line 147:
    expanded_key="${expanded_key/:hash:/$version_hash}"
                      ^-- SC3060 (warning): In POSIX sh, string replacement is undefined.

In ./setup.sh line 148:
    expanded_key="${expanded_key/:sha256:/$version_sha_256}"
                      ^-- SC3060 (warning): In POSIX sh, string replacement is undefined.

In ./setup.sh line 149:
    expanded_key="${expanded_key/:os:/$OS_NAME}"
                      ^---------------------------^ SC3060 (warning): In POSIX sh, string replacement is undefined.

In ./setup.sh line 158:
if [[ "$PRINT_ONLY" == true ]]; then
   ^-------------------------^ SC3010 (warning): In POSIX sh, [[ ]] is undefined.

In ./setup.sh line 165:
    if [[ "$TEST_MODE" == true ]]; then
           ^------------------------^ SC3010 (warning): In POSIX sh, [[ ]] is undefined.

In ./setup.sh line 189:
if [[ ! -x "$CACHE_PATH/bin/flutter" ]]; then
   ^-- SC3010 (warning): In POSIX sh, [[ ]] is undefined.

In ./setup.sh line 190:
    if [[ "$CHANNEL" == "master" || "$CHANNEL" == "main" ]]; then
           ^-- SC3010 (warning): In POSIX sh, [[ ]] is undefined.

In ./setup.sh line 192:
        if [[ "$VERSION" != "any" ]]; then
                   ^-----------------------^ SC3010 (warning): In POSIX sh, [[ ]] is undefined.

For more information:
  https://www.shellcheck.net/wiki/SC3010 -- In POSIX sh, [[ ]] is undefined.
  https://www.shellcheck.net/wiki/SC3014 -- In POSIX sh, == in place of = is ...
  https://www.shellcheck.net/wiki/SC3028 -- In POSIX sh, BASH_SOURCE is undef...

[bartekpacia]

This doesn't necessarily make sense since bash is a dependency of Flutter (link)