Build deployments on linux only

Evaluate deployments to {} (empty attribute set) for darwin since our
release.nix doesn't filter this out and hydra fails to evaluate this on
darin (cannot build nixos machines on darwin of course).

default.nix

@@ -91,9 +91,6 @@ rec {
 
   docs = import ./nix/docs.nix { inherit pkgs plutus; };
 
-  # Note: nix-build is not eager enough in evaluating `nix-build -A deployment` so 
-  # you need to specify a specific host. On hydra `pkgs.recurseIntoAttrs` solves
-  # this problem
   deployment = pkgs.recurseIntoAttrs (pkgs.callPackage ./deployment/morph {
     plutus = {
       plutus-docs = docs;

deployment/morph/default.nix

@@ -30,8 +30,9 @@ let
   # in a `pkgs.nixos` call to build the machine outside of morph.
   mkMachine = pkgs.callPackage ./mk-machine.nix { inherit plutus tfinfo; extraImports = [ fakeDeploymentOption ]; };
   buildMachine = { config, name }: (pkgs.nixos (mkMachine { inherit config name; })).toplevel;
+  linuxOnly = x: if pkgs.stdenv.isDarwin then { } else x;
 in
-import ./machines.nix {
+linuxOnly (import ./machines.nix {
   inherit pkgs tfinfo;
   mkMachine = buildMachine;
-}
+})