Specify bash `$(SHELL)` variable in Makefile when using `[[`

[mario-campos]

The Makefile utilizes a feature ,[[, of the Bash shell. Yet, it leaves the SHELL variable uninitialized, which means that make will default to sh, which does not support the bash-specific [[ keyword.

$ sh -c 'type [['
[[: not found
$ bash -c 'type [['
[[ is a shell keyword

This results in an awkward, preventable error:

🐋 ❯ make prepare NAME=bar REPOSITORY=github.com/mario-campos/pulumi-bar-native ORG=mario-campos
mv "provider/cmd/pulumi-resource-xyz" provider/cmd/pulumi-resource-bar # SED_SKIP
if [[ "Linux" != "Darwin" ]]; then \
        find . \( -path './.git' -o -path './sdk' \) -prune -o -not -name 'go.sum' -type f -exec sed -i '/SED_SKIP/!s,github.com/pulumi/pulumi-[x]yz,github.com/mario-campos/pulumi-bar-native,g' {} \; &> /dev/null; \
        find . \( -path './.git' -o -path './sdk' \) -prune -o -not -name 'go.sum' -type f -exec sed -i '/SED_SKIP/!s/[xX]yz/bar/g' {} \; &> /dev/null; \
        find . \( -path './.git' -o -path './sdk' \) -prune -o -not -name 'go.sum' -type f -exec sed -i '/SED_SKIP/!s/[aA]bc/mario-campos/g' {} \; &> /dev/null; \
fi
/bin/sh: 1: [[: not found
# In MacOS the -i parameter needs an empty string to execute in place.
if [[ "Linux" == "Darwin" ]]; then \
        find . \( -path './.git' -o -path './sdk' \) -prune -o -not -name 'go.sum' -type f -exec sed -i '' '/SED_SKIP/!s,github.com/pulumi/pulumi-[x]yz,github.com/mario-campos/pulumi-bar-native,g' {} \; &> /dev/null; \
        find . \( -path './.git' -o -path './sdk' \) -prune -o -not -name 'go.sum' -type f -exec sed -i '' '/SED_SKIP/!s/[xX]yz/bar/g' {} \; &> /dev/null; \
        find . \( -path './.git' -o -path './sdk' \) -prune -o -not -name 'go.sum' -type f -exec sed -i '' '/SED_SKIP/!s/[aA]bc/mario-campos/g' {} \; &> /dev/null; \
fi
/bin/sh: 1: [[: not found

How to fix? This PR sets the SHELL variable in the Makefile to /bin/bash to ensure that the [[ keyword is available.