fix(gemma): Fix build by backporting Elm 0.18

Backports an old version of the Elm language (before the release where
they made everyone rewrite their programs), from the same old nixpkgs
commit as was previously used in overrides (see CL/2646).

Change-Id: I0ae4cc611aa40269b290651ab982c1db93518d8a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2649
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

third_party/elmPackages_0_18/default.nix

@@ -0,0 +1,15 @@
+# Backports Elm packages for Elm 0.18 from an older channel of
+# nixpkgs.
+#
+# Elm 0.19 changed the language & package ecosystem completely,
+# essentially requiring a partial rewrite of all Elm apps. However,
+# //fun/gemma uses Elm 0.18 and I don't have time to rewrite it.
+
+{ pkgs, ... }:
+
+(import (pkgs.fetchFromGitHub {
+  owner = "NixOS";
+  repo = "nixpkgs";
+  rev = "14f9ee66e63077539252f8b4550049381a082518";
+  sha256 = "1wn7nmb1cqfk2j91l3rwc6yhimfkzxprb8wknw5wi57yhq9m6lv1";
+}) {}).elmPackages