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>

nix/buildGo/proto.nix

@@ -52,6 +52,8 @@ in rec {
     path = "google.golang.org/genproto";
     src = fetchGit {
       url = "https://github.com/google/go-genproto";
+      # necessary because https://github.com/NixOS/nix/issues/1923
+      ref = "main";
       rev = "83cc0476cb11ea0da33dacd4c6354ab192de6fe6";
     };