[ci.yaml] Fix osx_sdk cache name

[CaseyHillers]

flutter/flutter#85803

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide] and the [C++, Objective-C, Java style guides].
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See [testing the engine] for instructions on
  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the [CLA].
• All existing and new tests are passing.

[christopherfujino]

lolgtm

[jmagman]

For future reference we should avoid names like old_osx_sdk, there may be multiple sdks being used for different test scenarios, old and new aren't descriptive enough.

[CaseyHillers]

Thanks, I filed flutter/flutter#86515 to track this (i'm just trying to match the exact state in these PRs, but this makes sense to me as follow up).