[Reland] Enable Gradle cache for CI on single test target - #190635

This is a reland of #190474. Now using ':' - which should not break the validate luci test.

In https://cs.opensource.google/flutter/infra/+/main:config/lib/ci_yaml/ci_yaml.star;l=93-95 the swarming cache names are created. This is what broke on the last go - the cache names did not fit the regex: ^[a-z0-9_]+$

Before:
flutter_main_gradle_dists_8_4_bin, 8_13_rc_1_bin, 8_14_bin, 9_3_1_bin, 9_3_1_all ❌ does not match
After:
flutter_main_gradle_dists_8_4_bin_8_13_rc_1_bin_8_14_bin_9_3_1_bin_9_3_1_all ✅ does match

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [AI contribution guidelines] and understand my responsibilities, or I am not using AI tools.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• I followed the [breaking change policy] and added [Data Driven Fixes] where supported.
• All existing and new tests are passing.