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

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

testscripts/commands/config__cachedir.txt

@@ -0,0 +1,18 @@
+
+[windows] skip
+
+env HUGO_CACHEDIR=
+hugo config
+
+[darwin] stdout 'home/Library/Caches/hugo_cache'
+[linux] stdout 'xdgcachehome/hugo_cache'
+
+# Repeat it to make sure it handles an existing hugo_cache dir.
+hugo config
+
+[darwin] stdout 'home/Library/Caches/hugo_cache'
+[linux] stdout 'xdgcachehome/hugo_cache'
+
+-- hugo.toml --
+baseURL="https://example.com/"
+title="My New Hugo Site"