dogfood the most recent setup-dart action

[devoncarew]

The setup-dart action was recently re-written - ported from Bash to Dart. We should dogfood the latest commit in main to make sure that no functionality has regressed. To dogfood:

      - uses: dart-lang/setup-dart@8f7850f0f6056c5b91a1b9c206a53c9b1be28e4e

• dart-lang/ecosystem (switch to setup-dart from head ecosystem#82)
• dart-lang/linter (dogfood the new version of setup-dart linter#4113)
• dart-lang/test (dogfood the new setup-dart action test#1964)
• dart-lang/webdev (dogfood the new setup-dart action webdev#2017)
• dart-lang/build (dogfood the new version of setup-dart build#3468)
• dart-lang/tools

After the dogfooding is complete, we'll close this issue; at that time you can convert your use back to a regular dep (uses: dart-lang/setup-dart@v1).

[devoncarew]

v1.5.0 has been published and the v1 tag moved to point to v1.5.0.