cache: Set default cache path based on $USER

Change the default cache directory to `$TMPDIR/hugo_cache_$USER`, so
that multi-user systems do not have caches that interfere with each
other.  The other cache-choosing logic (e.g. Netlify exceptions,
configuration options) are not affected.

Fixes #7391

docs/content/en/commands/hugo_mod_verify.md

@@ -19,7 +19,7 @@ hugo mod verify [flags] [args]
 
 ```
   -b, --baseURL string      hostname (and path) to the root, e.g. https://spf13.com/
-      --cacheDir string     filesystem path to cache directory. Defaults: $TMPDIR/hugo_cache/
+      --cacheDir string     filesystem path to cache directory. Defaults: $TMPDIR/hugo_cache_$USER/
       --clean               delete module cache for dependencies that fail verification
   -c, --contentDir string   filesystem path to content directory
   -h, --help                help for verify