Fix pre-commit job pip cache config

[sbidoul]

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[sbidoul]

Please don't merge before testing on an affected repo. I'm not sure this is correct.

[SirPyTech]

Here a requirements.txt would not make sense as a cache key. Since we are in pre-commit, .pre-commit-config.yaml could be a good cache key.

Right: the cache is used for the libraries installed for pre-commit's hooks so it makes more sense to use its configuration as a cache key.

I'm not really convinced that using only the hook's versions is a good cache key because that would misbehave when a dependency of the hook is changed: in a new (local) environment the new dependency would be used, but the github action would use the cached versions?
Or maybe I should just look better into how the cache works for the github action.

It makes sense to use a requirements file as a cache key because it should have all the libraries with their versions pinned but there is no such thing for pre-commit that I know of.
Maybe we shouldn't use cache at all for pre-commit?