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

config/allconfig/allconfig.go

@@ -729,6 +729,7 @@ func fromLoadConfigResult(fs afero.Fs, logger loggers.Logger, res config.LoadCon
 	cfg := res.Cfg
 
 	all := &Config{}
+
 	err := decodeConfigFromParams(fs, bcfg, cfg, all, nil)
 	if err != nil {
 		return nil, err
@@ -871,6 +872,10 @@ func fromLoadConfigResult(fs afero.Fs, logger loggers.Logger, res config.LoadCon
 
 	bcfg.PublishDir = all.PublishDir
 	res.BaseConfig = bcfg
+	all.CommonDirs.CacheDir = bcfg.CacheDir
+	for _, l := range langConfigs {
+		l.CommonDirs.CacheDir = bcfg.CacheDir
+	}
 
 	cm := &Configs{
 		Base:                  all,