chore(bundler): Use the portable env shebang

This aligns with `resolve_dependencies.rb`.

Signed-off-by: Sebastian Schuberth <sebastian@doubleopen.org>

plugins/package-managers/bundler/src/main/resources/root_dependencies.rb

@@ -1,4 +1,4 @@
-#!/usr/bin/ruby
+#!/usr/bin/env ruby
 
 # Copyright (C) 2017 The ORT Project Authors (see <https://github.com/oss-review-toolkit/ort/blob/main/NOTICE>)
 #