hydra: no more git for firmware

hydra/firmware.nix

@@ -8,6 +8,8 @@ let
     srcPath = ".";
   }).override (oldArgs: {
     src = oldArgs.src.overrideAttrs (oldAttrs: {
+      # no more git, .git is dropped by Hydra
+      nativeBuildInputs = [];
       postPatch = ''
         VERSION="0.0-git"
         GITHASH="0000000000000000000000000000000000000000"