Use os.UserCacheDir as first fallback if cacheDir is not set

[bep]

We will now try

1. cacheDir (or, commonly set in environment as HUGO_CACHEDIR)
2. if on Netlify we use /opt/build/cache/hugo_cache/
3. os.UserCacheDir
4. A temp dir

Storing the cache, especially the module cache, in a temporary idea has had lots of hard to debug issues, especially on MacOS,
which this commit tries to fix.

This should also make it easier to locate the Hugo cache:

UserCacheDir returns the default root directory to use for user-specific cached data. Users should create their own
application-specific subdirectory within this one and use that.

On Unix systems, it returns $XDG_CACHE_HOME as specified by
https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html if non-empty, else $HOME/.cache. On Darwin, it
returns $HOME/Library/Caches. On Windows, it returns %LocalAppData%. On Plan 9, it returns $home/lib/cache.

If the location cannot be determined (for example, $HOME is not defined), then it will return an error.

Fixes #11286
Fixes #11291

[jmooring]

Please confirm that this is intended:

$ hugo config | grep -i cachedir
cachedir = '/home/jmooring/.cache/hugo_cache'

I just wanted to check because the typical pattern would be /home/jmooring/.cache/name_of_app.

[bep]

I just wanted to check because the typical pattern would be /home/jmooring/.cache/name_of_app.

That's not the pattern for cacheDir.

If you look at https://gohugo.io/getting-started/configuration/#configure-file-caches -- there's also a :project "token".

Anyway, the cacheDir logic is the same as before, just the root may be a little different.

[bep]

hugo config | grep -i cachedir

Also, all config keys in that list is lower case, so no need to -i.

[github-actions]

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