Improve brew doctor warnings about xattr and cask quarantine

[Rylan12]

Follow-up to #20151 and #20152

On Linux with a cask installed, brew doctor displays some unhelpful warnings about xattr and cask quarantine. This PR attempts to improve those.

Before:

$ brew doctor 
Warning: No Cask quarantine support available: there's no working version of `xattr` on this system.
/usr/bin/env: ‘/usr/bin/xattr’: No such file or directory

Warning: unknown xattr error: /usr/bin/env: ‘/usr/bin/xattr’: No such file or directory

After:

$ brew doctor
Warning: No Cask quarantine support available: not available on Linux.

Warning: Unable to find `xattr`.

Note: it might be better to not even run the check_cask_xattr if there is no quarantine support anyway, but I wasn’t 100% sure whether xattr was used for anything except quarantine.

Also, this PR was not made against the master branch because it depends on some of the work in #20152

[MikeMcQuaid]

Note: it might be better to not even run the check_cask_xattr if there is no quarantine support anyway, but I wasn’t 100% sure whether xattr was used for anything except quarantine.

Yeh, I think so. This message is still better than the status quo, though, so lets punt that to a follow-up.