git: Fix cross compilation

The problem was that Cargo didn't have a way to find a linker for the
build platform which is required to link the build script. So, when
cross compiling we add in the standard build -> build compiler so that
it can compile the build script.

This fix was taken from the ideas of @magicquark and @nwf in
https://github.com/NixOS/nixpkgs/issues/523378.

Fixes #523378

pkgs/by-name/gi/git/package.nix

@@ -212,6 +212,11 @@ stdenv.mkDerivation (finalAttrs: {
     libsecret
   ];
 
+  # This is required for building the rust build.rs script when cross compiling
+  depsBuildBuild = lib.optionals (stdenv.buildPlatform != stdenv.hostPlatform) [
+    buildPackages.stdenv.cc
+  ];
+
   env = {
     # required to support pthread_cancel()
     NIX_LDFLAGS =