Add support for gems installation.

Some tests use cocoapods which needs to be installed using gem and added
to the test execution environment.

Change-Id: Ia918afd86fcc55cd3d5bb04433d94e193f7b39a5
Bug: https://github.com/flutter/flutter/issues/65223
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/6440
Reviewed-by: Christopher Fujino <fujino@google.com>
Reviewed-by: Keyong Han <keyonghan@google.com>
Commit-Queue: Godofredo Contreras <godofredoc@google.com>