test_formulae: relax cache invalidation criteria

[carlocab]

We don't need to throw out our cached bottle if the only dependencies
that have changed are build- or test-only.

[MikeMcQuaid]

Some questions:

• does this not run the risk of the build/test cache having been poisoned?
• does this handle as expected the case where it's e.g. a build/test dependency of the current formula but a runtime dependency of a recursive dependency?

Good to merge once both of these are confidently 👍🏻

[github-actions]

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[carlocab]

• does this not run the risk of the build/test cache having been poisoned?

No more (or less) than the existing risk, since the Git hash check only verifies that dependencies haven't changed in Git since the cached bottles were last built.

• does this handle as expected the case where it's e.g. a build/test dependency of the current formula but a runtime dependency of a recursive dependency?

It should, yes.

[MikeMcQuaid]

Great, thanks for responding @carlocab!