Prefer locally installed /usr/bin/git

From what I understand, gLinux and Nix are sometimes compatible and sometimes
incompatible.

Some nix-installed programs rely on system state that differs from gLinux's
system state. In other cases, Google wraps existing programs (e.g. git) to
provide Google-specific configuration.

Ever since I switched to home-manager and set `programs.git.enable = true`,
magit, which uses the git that PATH exposes hasn't been able to push, fetch, and
a few other commands.

TODO(wpcarro): Define a base home.nix that my gLinux and NixOS machines can
extend.

configs/.config/nixpkgs/home.nix

@@ -41,6 +41,9 @@
 
   programs.git = {
     enable = true;
+    package = pkgs.writeShellScriptBin "git" ''
+      exec /usr/bin/git "$@"
+    '';
     userName = "William Carroll";
     userEmail = "wpcarro@gmail.com";
     aliases = {