Use absolute path to render 404
Use absolute path to render 404

Fixes Rails 6 deprecation:
DEPRECATION WARNING: render file: should be given the absolute path to a
file (called from block (2 levels) in render_not_found at
/home/travis/build/rubygems/rubygems.org/app/controllers/application_controller.rb:87
DEPRECATION WARNING: render file: should be given the absolute path to a
file (called from block (2 levels) in find_rubygem at
/home/travis/build/rubygems/rubygems.org/app/controllers/application_controller.rb:61)

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