Modify repo derivation to provide default local tool sources

The `repo` tool derivation by default includes only the main wrapper script
which fetches the actual tool sources from the internet. We modify the
derivation to provide default local sources patched with support for repo2nix,
unless specified otherwise with CLI parameters.

Author: jaen

flake.nix

@@ -25,6 +25,7 @@
 
     packages.x86_64-linux = {
       manual = (import ./docs { inherit pkgs; }).manual;
+      gitRepo = pkgs.gitRepo;
     };
 
     devShell.x86_64-linux = pkgs.mkShell {