Improve logical disk detection in WSL

[lmakarov]

Just ran into an edge case when the simplification done in
772f708#diff-41adca34c3eac75ca9453c7cb0634f6aL2496 fails.

/mnt/ in WSL has empty folders for all of the logical Windows drives that ever existed. If a drive is removed/disconnected (e.g. a USB disk) its letter is not cleaned up from /mnt/.

$ ls /mnt/
c  d

fin then tries to mount a non-existing drive:

Adding docker SMB share docksal-d...
mount: special device //192.168.64.1/docksal-d does not exist
exit status 32
              Mount command failed... Trying an alternative method...
mount: wrong fs type, bad option, bad superblock on //192.168.64.1/docksal-d,
       missing codepage or helper program, or other error
       (for several filesystems (e.g. nfs, cifs) you might
       need a /sbin/mount.<type> helper program)

       In some cases useful info is found in syslog - try
       dmesg | tail or so.
exit status 32
               ERROR:  SMB share mount failed.
         Make sure your Windows password is correct and run fin vm mount to try again.
         For troubleshooting instructions see https://docs.docksal.io/troubleshooting/windows-smb/

The older method, while not being elegant, is far for reliable:

$ wmic.exe logicaldisk get drivetype,name | grep 3 | awk '{print $2}' | sed 's/\r//g'
C:

[lmakarov]

Fixed in d433e24