Update ruby.hcl

Set GEM_PATH to be the same as GEM_HOME so that gems are bundled hermetically with the repo using ruby.

ruby.hcl

@@ -4,6 +4,7 @@ binaries = ["bin/*"]
 test = "ruby --version"
 env = {
   "GEM_HOME": "${HERMIT_ENV}/.hermit/ruby/gem",
+  "GEM_PATH": "${HERMIT_ENV}/.hermit/ruby/gem",
   "PATH": "${HERMIT_ENV}/.hermit/ruby/gem/bin:${PATH}",
 }