ci(gemfile): allow rubygems proxy to be provided as an env var [skip ci]

* Automated using https://github.com/myii/ssf-formula/pull/408

Gemfile

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-source 'https://rubygems.org'
+source ENV['PROXY_RUBYGEMSORG'] || 'https://rubygems.org'
 
 # Install the `inspec` gem using `git` because versions after `4.22.22`
 # suppress diff output; this version fixes this for our uses.