chruby: allow for specifying Rubies to use

This allows the user to specify an attrset of Rubies to use with chruby by
overriding the default of ~/.rubies with a path to a symlink tree.

pkgs/development/tools/misc/chruby/default.nix

@@ -1,6 +1,13 @@
-{ stdenv, lib, fetchFromGitHub }:
+{ stdenv, lib, fetchFromGitHub, runCommand, rubies ? null }:
 
-stdenv.mkDerivation rec {
+let
+  rubiesEnv = runCommand "chruby-env" { preferLocalBuild = true; } ''
+    mkdir $out
+    ${lib.concatStrings
+        (lib.mapAttrsToList (name: path: "ln -s ${path} $out/${name}\n") rubies)}
+  '';
+
+in stdenv.mkDerivation rec {
   name = "chruby";
 
   src = fetchFromGitHub {
@@ -10,7 +17,15 @@ stdenv.mkDerivation rec {
     sha256 = "1iq9milnnj3189yw02hkly2pnnh4g0vn2fxq6dfx90kldjwpwxq5";
   };
 
-  phases = [ "unpackPhase" "installPhase" "fixupPhase" ];
+  phases = [ "unpackPhase" "patchPhase" "installPhase" "fixupPhase" ];
+
+  patches = lib.optionalString (rubies != null) [
+    ./env.patch
+  ];
+
+  postPatch = lib.optionalString (rubies != null) ''
+    substituteInPlace share/chruby/chruby.sh --replace "@rubiesEnv@" ${rubiesEnv}
+  '';
 
   installPhase = ''
     mkdir $out