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/lisp/cl-plus-ssl.nix

@@ -3,9 +3,10 @@
 
 with depot.nix;
 
-let src = builtins.fetchGit {
+let src = pkgs.fetchgit {
   url = "https://github.com/cl-plus-ssl/cl-plus-ssl.git";
   rev = "29081992f6d7b4e3aa2c5eeece4cd92b745071f4";
+  hash = "sha256:16lyrixl98b7vy29dbbzkbq0xaz789350dajrr1gdny5i55rkjq0";
 };
 in buildLisp.library {
   name = "cl-plus-ssl";