Merge #6885

6885: Don't check for the existence of a writable home directory if BUNDLE_USER_HOME is set r=deivid-rodriguez a=jturkel

#6024 added support for specifying an alternate bundler home directory via the `BUNDLE_USER_HOME` environment variable. Unfortunately bundler still checks for a writable user home directory even if the `BUNDLE_USER_HOME` override is specified resulting in warnings like this:
```
`/home/appuser` is not a directory.
Bundler will use `/tmp/bundler/home/unknown' as your home directory temporarily.
```
The fix for this problem is to only evaluate bundler path fallbacks if environment variable overrides are not specified.

Co-authored-by: Joel Turkel <jturkel@salsify.com>