macvim: fix build after Darwin SDK update

MacVim needs to use the system Xcode to build. Having `DEVELOPER_DIR`
set in the environment causes Xcode to try to use the nixpkgs SDK, which
doesn’t work. Unsetting it allows MacVim to build.

pkgs/applications/editors/vim/macvim.nix

@@ -94,6 +94,8 @@ stdenv.mkDerivation (finalAttrs: {
       cppflags = map (drv: "-isystem ${lib.getDev drv}/include") inputs;
     in
     ''
+      unset DEVELOPER_DIR # Use the system Xcode not the nixpkgs SDK.
+
       CC=/usr/bin/clang
 
       DEV_DIR=$(/usr/bin/xcode-select -print-path)/Platforms/MacOSX.platform/Developer