nixos: Fix detection of btrfs root volume

Regression introduced by 801c920e

.

Since then, the btrfsSimple subtest of the installer VM test fails with:

Btrfs did not return a path for the subvolume at /

The reason for this is that the output for "btrfs subvol show" has
changed between version 4.8.2 and 4.13.1.

For example the output of "btrfs subvol show /" in version 4.8.2 was:

/ is toplevel subvolume

In version 4.13.1, the output now is the following and thus the regular
expressions used in nixos-generate-config.pl and install-grub.pl now
match (which results in the error mentioned above):

/
        Name:                   <FS_TREE>
        UUID:                   -
        Parent UUID:            -
        Received UUID:          -
        Creation time:          -
        Subvolume ID:           5
        Generation:             287270
        Gen at creation:        0
        Parent ID:              0
        Top level ID:           0
        Flags:                  -
        Snapshot(s):

In order to fix this I've changed nixos-generate-config.pl and
install-grub.pl, because both use "btrfs subvol show" in a similar vein,
so the regex for parsing the output now doesn't match anymore whenever
the volume path is "/", which should result in the same behaviour as we
had with btrfs-progs version 4.8.2.

Tested against the btrfsSimple, btrfsSubvols and btrfsSubvolDefault
subtests of the installer VM test and they all succeed now.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>