Use /usr/bin/env ruby shebang

Some plugins are broken when running on a system with `EMBEDDED_RUBY=true` where there is no system ruby installed, only the embedded ruby. The problem is due to the use of hardcoded `/usr/bin/ruby` shebang lines, resulting in the check failing because '$plugin: not found'.

This PR trivially fixes the 2 affected plugins: `check-raid.rb` and `check-hardware-fail.rb`.