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/golang.org/x/sys/default.nix

@@ -1,9 +1,11 @@
-{ depot, ... }:
+{ depot, pkgs, ... }:
 
 depot.nix.buildGo.external {
   path = "golang.org/x/sys";
-  src = builtins.fetchGit {
+
+  src = pkgs.fetchgit {
     url = "https://go.googlesource.com/sys";
     rev = "ac6580df4449443a05718fd7858c1f91ad5f8d20";
+    hash = "sha256:14gvx65w5lddi20s4wypbbvbg9ni3m8777jhp9nqxhixc61k3dyi";
   };
 }