Please, fix CI caching

[Anton-Latukha]

#2503 had a number of changes into the codebase, project configuration & CI.

The caching is not configured to run inside PRs, so the merge of the PR revealed the breakage in the workflow.

I'm taking this task to fix it.

[Anton-Latukha]

Currently, it seem that the:

Failed to build stylish-haskell-0.13.0.0.
Build log ...

https://github.com/haskell/haskell-language-server/runs/4802586523?check_suite_focus=true

So next step is to establish what is there.

[Anton-Latukha]

Addressed in: #2567 (comment).