I also had fun substantiating the claim "86% of actions/setup-go test runs are unnecessary." We had a general sense things were faster, and we measured impacts immediately after we made changes, but hard to understand long-run performance vs. a counterfactual.

The trick was to run back over our git history and calculate, for each commit,

1. The test package Go cache keys at that point

2. The GitHub actions/cache keys constructed by actions/setup-go and cloudx-io/setup-go respectively

Once you have these mappings, you can

1. Pick some arbitrary HEAD commit

2. Model which prior GitHub cache blob would be loaded under each action

3. Compare the test package Go cache keys in that loaded blob against those for HEAD to determine which test packages would run vs. skip

Might write this up in greater depth sometime soon.