Setup tweaks

[DingDongSoLong4]

Two tweaks to the initial setup process:

• When a config file is specified on the command line or using STASH_CONFIG_FILE, the final confirmation screen now correctly shows paths relative to the config file location - previously they'd just show as relative to /. I missed this in Improved/fixed macOS support #4153.
• Overridden paths (ie with an environment variable) are now hidden from the final confirmation screen. Even though you aren't able to set values for overridden paths, they'd previously still show in the final screen with deceptive values, as shown in the screenshot below.

Both of the following were run using STASH_CACHE=/tmp ./stash -c stash/config.yml.

Before:

Now: