nixos: Fix detection of btrfs root volume

Regression introduced by 801c920e95642ec663d4aa486c81e5443e25b182.

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 <[e-mail]>

nixos/modules/installer/tools/nixos-generate-config.pl

···
         if ($status != 0 || join("", @info) =~ /ERROR:/) {
             die "Failed to retrieve subvolume info for $mountPoint\n";
         }
-        my @ids = join("", @info) =~ m/Subvolume ID:[ \t\n]*([0-9]*)/;
+        my @ids = join("\n", @info) =~ m/^(?!\/\n).*Subvolume ID:[ \t\n]*([0-9]+)/s;
         if ($#ids > 0) {
             die "Btrfs subvol name for $mountPoint listed multiple times in mount\n"
         } elsif ($#ids == 0) {

nixos/modules/system/boot/loader/grub/install-grub.pl

···
                 if ($status != 0) {
                     die "Failed to retrieve subvolume info for @{[$fs->mount]}\n";
                 }
-                my @ids = join("", @id_info) =~ m/Subvolume ID:[ \t\n]*([^ \t\n]*)/;
+                my @ids = join("\n", @id_info) =~ m/^(?!\/\n).*Subvolume ID:[ \t\n]*([0-9]+)/s;
                 if ($#ids > 0) {
                     die "Btrfs subvol name for @{[$fs->device]} listed multiple times in mount\n"
                 } elsif ($#ids == 0) {