Cache black formatting information across CI runs. (#9413)

Save and persist `black`'s formatted files cache across CI runs.

Around a ~20s win, 21s -> 2s. Most cases should be close to this best
case scenario, since most PRs don't modify most files — and this PR
makes sure we don't re-check files that haven't changed.

Before:

![image](https://github.com/langchain-ai/langchain/assets/2348618/6c5670c5-be70-4a18-aa2a-ece5e4425d1e)

After:

![image](https://github.com/langchain-ai/langchain/assets/2348618/37810d27-c611-4f76-b9bd-e827cefbaa0a)