Use `pack build --force-color` in CI instead of `script` TTY workaround

[edmorley]

As of Pack 0.33.0 (which this repo is using thanks to #462), there is a new --force-color argument that makes pack build output logs in colour even if a TTY wasn't detected (as is the case on GHA): https://github.com/buildpacks/pack/releases/tag/v0.33.0

This makes the script -e -c TTY workaround unnecessary. (It's also not needed for any direct Cargo usages, since they are covered by the existing CARGO_TERM_COLOR: always.)

I've also:

• removed a redundant cargo build step (run does a build, and the Cargo compilation output can be suppressed using --quiet)
  • replaced a manual git clone with actions/checkout, since it handles things like retries, faster shallow clones etc.

GUS-W-14968745.

[edmorley]

Before: https://github.com/heroku/buildpacks-ruby/actions/runs/7773089782/job/21196326133#step:12:30 https://github.com/heroku/buildpacks-ruby/actions/runs/7773089782/job/21196326188#step:8:17

After: https://github.com/heroku/buildpacks-ruby/actions/runs/7784979705/job/21226560938?pr=255#step:12:29 https://github.com/heroku/buildpacks-ruby/actions/runs/7784979705/job/21226562513?pr=255#step:7:14