Fix handling binaries with spaces in the path

[James McMurray]

It seems the addition of shellexpand broke the handling of spaces in the path for the exec command.

e.g.

$ vopono exec --user plex --custom ~/Downloads/wg-RO-9.conf --protocol wireguard --provider custom --custom-port-forwarding protonvpn /usr/lib/plexmediaserver/Plex\ Media\ Server

Now fails.

[James McMurray]

Fixed in #264