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/env.patch

@@ -0,0 +1,12 @@
+diff --git a/share/chruby/chruby.sh b/share/chruby/chruby.sh
+--- a/share/chruby/chruby.sh
++++ b/share/chruby/chruby.sh
+@@ -1,7 +1,7 @@
+ CHRUBY_VERSION="0.3.8"
+ RUBIES=()
+ 
+-for dir in "$PREFIX/opt/rubies" "$HOME/.rubies"; do
++for dir in @rubiesEnv@; do
+ 	[[ -d "$dir" && -n "$(ls -A "$dir")" ]] && RUBIES+=("$dir"/*)
+ done
+ unset dir