Make it possible to pipe setup.sh to shell and make it independent of GHA

[bartekpacia]

The setup.sh is a core of this action, and it should be relatively easy to make it GitHub Actions-independent.

Then this script could be easily used e.g. when doing one-off Flutter install, or building a container image:

 curl -fsSL https://raw.githubusercontent.com/subosito/flutter-action/v2/setup.sh | bash -s -- -n '3.19.0' -a 'arm64' -c "$PWD" 'stable'

Relevant StackOverflow question: Passing parameters to bash when executing a script fetched by curl

Different takes on whether piping curl to shell is a good idea:

• Piping curl to s(hell)
• Curl to shell isn’t so bad