installer/nixos-generate-config: correctly detect bcache

PR #256638 inadvertently introduced a bug in `nixos-generate-config` whereby it
would never put `bcache` into the `availableKernelModules` for the initrd.

This is because the `qr` operator in Perl returns a regex object, rather than
matching it; the regex object evaluates to true, making the filter expression
effectively `grep(!true, @bcacheDevices)`, which will always return an empty
list.

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

@@ -257,7 +257,7 @@ foreach my $path (glob "/sys/class/{block,mmc_host}/*") {
 
 # Add bcache module, if needed.
 my @bcacheDevices = glob("/dev/bcache*");
-@bcacheDevices = grep(!qr#dev/bcachefs.*#, @bcacheDevices);
+@bcacheDevices = grep(!m#dev/bcachefs.*#, @bcacheDevices);
 if (scalar @bcacheDevices > 0) {
     push @initrdAvailableKernelModules, "bcache";
 }