Add support for Rubinius

* Only works on ubuntu-latest.

ruby-install-builder.js

@@ -17,7 +17,12 @@ export async function getAvailableVersions(engine) {
 
 export async function install(platform, ruby) {
   const rubyPrefix = await downloadAndExtract(platform, ruby)
+
   core.addPath(path.join(rubyPrefix, 'bin'))
+  if (ruby.startsWith('rubinius')) {
+    core.addPath(path.join(rubyPrefix, 'gems', 'bin'))
+  }
+
   return rubyPrefix
 }