Change base cache dir

[ErikDanielsson]

The base cache directory was changed in a previous PR be to $XDG_CONFIG_DIR or .config. In my opinion it makes more sense to use $XDG_CACHE_DIR or .cache.

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

[codecov]

Codecov Report

Merging #1636 (9bb817a) into dev (5484f39) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              dev    #1636   +/-   ##
=======================================
  Coverage   64.39%   64.39%           
=======================================
  Files          54       54           
  Lines        6299     6299           
=======================================
  Hits         4056     4056           
  Misses       2243     2243           

Impacted Files	Coverage Δ
nf_core/utils.py	80.32% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5484f39...9bb817a. Read the comment docs.

[fabianegli]

Looks nice. Did you check if there is the potential to adopt basedir-spec in other places in this code base?

Not sure what we do, but do we need to alarm users about a stale cache after the change?

[ErikDanielsson]

I don't think we have any other persistent files than the caches between sessions, so this should be the only place needing to change currently.

Good idea, I will look into how we did it the last time.

[ewels]

LGTM 👍🏻

[ewels]

Not too fussed about the stale files. We're going to use the same path in a moment for modules stuff so it will be subfiles in an active subdir. They shouldn't cause too many problems, I think telling people about them will cause more problems 😆