nixos-generate-config: substitute bcachefs

For systems where the package is marked as broken, use `false` instead.
The Perl script checks for a non-zero exit status, so it will handle it
as if the command failed.

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

@@ -505,7 +505,7 @@ EOF
     # This should work for single and multi-device systems.
     # still needs subvolume support
     if ($fsType eq "bcachefs") {
-        my ($status, @info) = runCommand("bcachefs fs usage $rootDir$mountPoint");
+        my ($status, @info) = runCommand("@bcachefs@ fs usage $rootDir$mountPoint");
         my $UUID = $info[0];
 
         if ($status == 0 && $UUID =~ /^Filesystem:[ \t\n]*([0-9a-z-]+)/) {

nixos/modules/installer/tools/tools.nix

@@ -38,6 +38,11 @@ let
       );
       hostPlatformSystem = pkgs.stdenv.hostPlatform.system;
       detectvirt = lib.getExe' config.systemd.package "systemd-detect-virt";
+      bcachefs =
+        if pkgs.bcachefs-tools.meta.broken then
+          lib.getExe' pkgs.coreutils "false"
+        else
+          lib.getExe pkgs.bcachefs-tools;
       btrfs = lib.getExe pkgs.btrfs-progs;
       inherit (config.system.nixos-generate-config) configuration desktopConfiguration flake;
       xserverEnabled = config.services.xserver.enable;