Consistently use HOMEBREW_RUBY_WARNINGS when calling ruby

[dawidd6]

This allows us to suppress warnings like:

/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/language/python.rb:8: warning: Insecure world writable dir /home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/shims/scm in PATH, mode 040777

Same option is passed in dev-cmd/test.rb:

brew/Library/Homebrew/dev-cmd/test.rb

Line 81 in 8041649

-W0

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?

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