chromium: use python3 for build to work towards working cross

pkgs/applications/networking/browsers/chromium/common.nix

@@ -55,7 +55,7 @@
 buildFun:
 
 let
-  python3WithPackages = python3.withPackages(ps: with ps; [
+  python3WithPackages = python3.pythonForBuild.withPackages(ps: with ps; [
     ply jinja2 setuptools
   ]);
   clangFormatPython3 = fetchurl {
@@ -314,7 +314,7 @@ let
 
       # This is to ensure expansion of $out.
       libExecPath="${libExecPath}"
-      ${python3}/bin/python3 build/linux/unbundle/replace_gn_files.py --system-libraries ${toString gnSystemLibraries}
+      ${python3.pythonForBuild}/bin/python3 build/linux/unbundle/replace_gn_files.py --system-libraries ${toString gnSystemLibraries}
       ${gnChromium}/bin/gn gen --args=${lib.escapeShellArg gnFlags} out/Release | tee gn-gen-outputs.txt
 
       # Fail if `gn gen` contains a WARNING.