artiq-fast: add strictDeps=true to fetchcargo.nix

part of gitea issue #34. unfortunately, the flag has inferior
documentation, so this is just a try.

artiq-fast/fetchcargo.nix

@@ -3,6 +3,7 @@
 stdenv.mkDerivation {
   name = "${name}-vendor";
   nativeBuildInputs = [ cacert git cargo cargo-vendor ];
+  strictDeps = true;
   inherit src;
 
   phases = "unpackPhase patchPhase installPhase";