Allow starting a shell on stage 1 failure

When people copy the ISO to USB it can happen that the root device isn't known. Having a shell greatly improves that situation.

nixos/modules/installer/cd-dvd/iso-image.nix

@@ -176,7 +176,10 @@ in
     # UUID of the USB stick.  It would be nicer to write
     # `root=/dev/disk/by-label/...' here, but UNetbootin doesn't
     # recognise that.
-    boot.kernelParams = [ "root=LABEL=${config.isoImage.volumeID}" ];
+    boot.kernelParams =
+      [ "root=LABEL=${config.isoImage.volumeID}"
+        "boot.shell_on_fail"
+      ];
 
     fileSystems."/" =
       { fsType = "tmpfs";