buildMozillaMach: migrate into build-support

The builder has multiple consumers, so it is fine to separate it from
Firefox, but the main motivation is to separate code ownership between
the builder and individual firefox versions.

ci/OWNERS

@@ -234,6 +234,7 @@ nixos/modules/installer/tools/nix-fallback-paths.nix  @NixOS/nix-team @raitobeza
 /nixos/tests/snapcast.nix @mweinelt
 
 # Browsers
+/pkgs/build-support/build-mozilla-mach @mweinelt
 /pkgs/applications/networking/browsers/librewolf @squalus @DominicWrege @fpletz @LordGrimmauld
 /pkgs/applications/networking/browsers/firefox @mweinelt
 /pkgs/applications/networking/browsers/chromium @emilylange @networkException

pkgs/top-level/all-packages.nix

@@ -12185,7 +12185,7 @@ with pkgs;
   };
 
   buildMozillaMach =
-    opts: callPackage (import ../applications/networking/browsers/firefox/common.nix opts) { };
+    opts: callPackage (import ../build-support/build-mozilla-mach/default.nix opts) { };
 
   firefox-unwrapped = import ../applications/networking/browsers/firefox/packages/firefox.nix {
     inherit