fix(3p): Move away from builtins.fetchGit

Moves to the derivation-based git fetchers everywhere in third-party.
This might help with forward-compatibility with newer Nix versions,
though that's not our primary concern right now.

Change-Id: I565bb72585b8639893e9ea3a9e233338aede63a9
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3903
Tested-by: BuildkiteCI
Reviewed-by: zseri <zseri.devel@ytrizja.de>

third_party/gopkgs/google.golang.org/genproto/default.nix

@@ -1,10 +1,12 @@
-{ depot, ... }:
+{ depot, pkgs, ... }:
 
 depot.nix.buildGo.external {
   path = "google.golang.org/genproto";
-  src = builtins.fetchGit {
+
+  src = pkgs.fetchgit {
     url = "https://github.com/google/go-genproto";
     rev = "0243a4be9c8f1264d238fdc2895620b4d9baf9e1";
+    hash = "sha256:071672lk0pzns98ncbqk6np7l9flwh84hjjibhhm2s1fi941m6q3";
   };
 
   deps = with depot.third_party; [