[rubygems/rubygems] Improve command suggestion when gems are missing

Previous attempt to use a full path when the running version is
different than the one that would be activated by default are different
was not correct.

This is because if this error happens in a `bundle exec` context, the
`Gem.loaded_specs` is cleared because we're in an exec'd process, so
will be always using a full path in these cases.

This alternative approach should do what I was expecting.

rubygems/rubygems@e0bb8bf51b

lib/bundler/setup.rb

@@ -12,7 +12,9 @@
       Bundler.ui.error e.message
       Bundler.ui.warn e.backtrace.join("\n") if ENV["DEBUG"]
       if e.is_a?(Bundler::GemNotFound)
-        suggested_bundle = Gem.loaded_specs["bundler"] ? "bundle" : Bundler::SharedHelpers.bundle_bin_path
+        default_bundle = Gem.bin_path("bundler", "bundle")
+        current_bundle = Bundler::SharedHelpers.bundle_bin_path
+        suggested_bundle = default_bundle == current_bundle ? "bundle" : current_bundle
         suggested_cmd = "#{suggested_bundle} install"
         original_gemfile = Bundler.original_env["BUNDLE_GEMFILE"]
         suggested_cmd += " --gemfile #{original_gemfile}" if original_gemfile