Cache downloaded swift packages

[theospears]

Summary

Speed up CI by using the cache action to save packages downloaded by SPM.

Validation

Verify this PR runs CI, and then uses cache the second time.

[krugerk]

https://github.com/actions/cache/blob/main/examples.md#swift---swift-package-manager

Shows how to use it for typical setups:

- uses: actions/cache@v4
  with:
    path: .build
    key: ${{ runner.os }}-spm-${{ hashFiles('**/Package.resolved') }}
    restore-keys: |
      ${{ runner.os }}-spm-

[theospears]

https://stackoverflow.com/questions/68081067/spm-cache-not-working-on-github-actions-any-ideas suggests that is true if you are using swift package directly, but not if the resolution is being done by xcodebuild, as is being done here

[krugerk]

OK so we specify cloned_source_packages_path in scan/run_tests and replace path: .build with the path specified in run_tests' cloned_source_packages_path parameter.

https://itnext.io/effective-swift-package-manager-caching-with-fastlane-and-github-actions-b241585f0a4c

[theospears]

I looks like specifying derived_data_path also works. Do you have strong opinions on which is better?

[krugerk]

I lean toward the cloned source option rather than the entire derived data. It seems more precisely targeted to what we want.

[krugerk]

And it seems CI does not take long at all to grab the spm managed packages currently.

11:55:31
11:54:58

🤞 the direct cache would be faster. It probably would be more efficient in any case.