treewide: Add lots of meta.platforms

Build-tested on x86_64 Linux & Mac.

pkgs/shells/fish-foreign-env/default.nix

@@ -24,5 +24,6 @@ stdenv.mkDerivation rec {
     description = "A foreign environment interface for Fish shell";
     license = licenses.mit;
     maintainers = with maintainers; [ jgillich ];
+    platforms = with platforms; unix;
   };
 }

pkgs/shells/zsh-prezto/default.nix

@@ -41,5 +41,6 @@ in stdenv.mkDerivation rec {
     homepage = "https://github.com/sorin-ionescu/prezto";
     license = licenses.mit;
     maintainers = with maintainers; [ garbas ];
+    platforms = with platforms; unix;
   };
 }