Snapd installs bash completion files from snaps in
`/usr/share/bash-completion/completions` which in some distributions
is a read-only filesystem.
Instead of installing them in `/usr` we can install them within
`/var/lib/snapd` which should always be writable.
Because `/var/lib/snapd/desktop` is already in `XDG_DATA_DIRS`, we can
save the files there.
Because bash-completion 2.1 and prior do not support `XDG_DATA_DIRS`,
on older distributions, the legacy path
`/usr/share/bash-completion/completions` will still be used.
