Point PROTOC to nixpkgs.protobuf pkg in shell.nix (#15931)

This fixes an issue on NixOS where Zed's proto crate fails to build.
Cargo expects to find protoc in the Cargo registry, but due to the
distro's non-standard filesystem this expectation is invalid.

Release Notes:

- N/A

shell.nix

@@ -52,5 +52,6 @@ else
           "assets/fonts/zed-sans"
         ];
       };
+      PROTOC = "${pkgs.protobuf}/bin/protoc";
     };
   }