ci(all): use GH Action setup-flutter to install Flutter instead of custom script

[miquelbeltran]

Description

Seems that CI installs of Flutter, by cloning the main repo, are currently broken. See flutter/flutter#163198

Switching to use the https://github.com/flutter-actions/setup-flutter action instead of our custom install script, as the process seems now more complex.

Related Issues

Related: flutter/flutter#163198

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I titled the PR using Conventional Commits.
• I did not modify the CHANGELOG.md nor the plugin version in pubspec.yaml files.
• All existing and new tests are passing.
• The analyzer (flutter analyze) does not report any problems on my PR.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate that with a ! in the title as explained in Conventional Commits).
• No, this is not a breaking change.

[miquelbeltran]

Going ahead and merging this since the CI is completely broken otherwise and it stops other PRs