Workaround for shebang length limit

Ruby executable is normally available in gem_executable shebang
but shebang's length is limited to 128 bytes (Linux < 5.1)
so it can not be used for very long executable path.

rubygems.py

@@ -200,6 +200,7 @@ class Recipe(object):
 
     def _install_gem(self, gemname, gem_executable, bindir):
         cmd = [
+            self.ruby_executable,
             gem_executable,
             'install',
             '--no-rdoc',