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/github.com/golang/groupcache/default.nix

@@ -1,9 +1,15 @@
-{ depot, ... }:
+{ depot, pkgs, ... }:
 
 depot.nix.buildGo.external {
   path = "github.com/golang/groupcache";
-  src = builtins.fetchGit {
+
+  src = pkgs.fetchgit {
     url = "https://github.com/golang/groupcache";
     rev = "611e8accdfc92c4187d399e95ce826046d4c8d73";
+    hash = "sha256:0ydaq1xn03h2arfdri0vcv0df19pk8dvq4ly5hm1kv18yjfv1v13";
   };
+
+  deps = with depot.third_party; [
+    gopkgs."github.com".golang.protobuf.proto
+  ];
 }