feat: add dependency derivation to flake output, so it can be pinned

If you write nix build .#deps ,then the result link gets added
as a gc root so it doesn't get garbage collected.

flake.nix

@@ -119,6 +119,7 @@
 
         packages = {
           default = my-crate;
+          deps = cargoArtifacts;
         } // lib.optionalAttrs (!pkgs.stdenv.isDarwin) {
           my-crate-llvm-coverage = craneLibLLvmTools.cargoLlvmCov (commonArgs // {
             inherit cargoArtifacts;