resholve: use originalSrc for nixpkgs-update bot

Effort to fix automatic nixpkgs-update updates for resholved
packages in 9f6310d did help the bot get further, but it
then failed to find the source outputHash (the outer
derivation's source is the inner derivation; bot looks for
outer.src.outputHash; ours is at outer.src.src.outputHash).

This change uses `originalSrc` to indicate the source of the
inner derivation. Along with ryantm/nixpkgs-update#324, this
enables the bot to fall back on an attr that Nix/nixpkgs are
not directly depending on, supporting automatic updates for
packages built with `resholve.mkDerivation`.

pkgs/development/misc/resholve/resholve-utils.nix

@@ -185,6 +185,8 @@ rec {
       # retain a reference to the base
       passthru = unresholved.passthru // {
         unresholved = unresholved;
+        # fallback attr for update bot to query our src
+        originalSrc = unresholved.src;
       };
 
       # do these imply that we should use NoCC or something?