install-grub: Make more robust against GC'ed system profile entries

Previously this barfed with:

  updating GRUB 2 menu...
  fileparse(): need a valid pathname at /nix/store/zldbbngl0f8g5iv4rslygxwp0dbg1624-install-grub.pl line 391.
  warning: error(s) occured while switching to the new configuration

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

@@ -387,6 +387,10 @@ sub addProfile {
     my $curEntry = 0;
     foreach my $link (@links) {
         last if $curEntry++ >= $configurationLimit;
+        if (! -e "$link/nixos-version") {
+            warn "skipping corrupt system profile entry ‘$link’\n";
+            next;
+        }
         my $date = strftime("%F", localtime(lstat($link)->mtime));
         my $version =
             -e "$link/nixos-version"