Cookiecutter Tests and pre-commit workflows should use separate caches

[cjolowicz]

pre-commit environments are cached in the Tests and pre-commit workflows of the Cookiecutter. Caches are scoped to the key and branch, and both workflows use the same cache key format. These environments are quite different, however; the one in the Tests workflow is for generated projects and only has Prettier, while the one in the pre-commit workflow is for the Cookiecutter and has both Prettier and pre-commit-hooks. We should either differentiate the cache keys, or drop caching (see #640).

[cjolowicz]

They would only use the same cache if the .pre-commit-config.yaml were identical (in which case it would be fine).