firefox*: fixup build after updating sed

I failed to simply patch the bad [:space:] usage; I don't know why.

pkgs/applications/networking/browsers/firefox/default.nix

@@ -44,7 +44,7 @@ common = { pname, version, sha512, updateScript }: stdenv.mkDerivation rec {
     ++ lib.optional enableGTK3 gtk3
     ++ lib.optionals (!passthru.ffmpegSupport) [ gstreamer gst_plugins_base ];
 
-  nativeBuildInputs = [autoconf213 which];
+  nativeBuildInputs = [ autoconf213 which gnused ];
 
   configureFlags =
     [ "--enable-application=browser"

pkgs/top-level/all-packages.nix

@@ -13216,6 +13216,7 @@ in
     libpng = libpng_apng;
     enableGTK3 = false;
     python = python2;
+    gnused = gnused_422;
   }) firefox-unwrapped firefox-esr-unwrapped;
 
   firefox = wrapFirefox firefox-unwrapped { };