fix(gemma): Almost fix Gemma build by porting an old Elm

This pulls in an old version of Elm from NixOS 17.09 which can still
build the Elm code in Gemma.

However, the Common Lisp build is now broken in some other way.

default.nix

@@ -12,7 +12,7 @@ let
     tazjin = {
       blog = import ./services/tazblog { inherit pkgs; };
       blog_cli = pkgs.callPackage ./tools/blog_cli {};
-      gemma = import ./services/gemma { inherit pkgs; };
+      gemma = pkgs.callPackage ./services/gemma {};
     };
 
     # Third-party projects (either vendored or modified from nixpkgs) go here:
@@ -25,6 +25,15 @@ let
       rev = "a7076c0f83e5c06ea9067b71171859fa2ba8afd9";
       sha256 = "1p4n2ja4ciw3qfskn65ggpy37mvgf2sslxqmqn8s8jjarnqcyfny";
     }) { pkgs = super; };
+
+    # Gemma needs an older version of Elm to be built. Updating it to
+    # the newer version is a lot of effort.
+    elmPackages = (import (super.fetchFromGitHub {
+      owner = "NixOS";
+      repo = "nixpkgs";
+      rev = "14f9ee66e63077539252f8b4550049381a082518";
+      sha256 = "1wn7nmb1cqfk2j91l3rwc6yhimfkzxprb8wknw5wi57yhq9m6lv1";
+    }) {}).elmPackages;
   };
 
   # The pinned commit here is identical to the public nixery.dev