Fix gem install with --platform flag not matching simulated platform correctly

[deivid-rodriguez]

What was the end-user or developer problem that led to this PR?

$ gem install sorbet-static -v 0.5.5460 --platform universal-darwin-19
ERROR:  Could not find a valid gem 'sorbet-static' (= 0.5.5460) in any repository
ERROR:  Possible alternatives: sorbet-static

What is your fix for the problem, implemented in this PR?

Make sure platform matching uses Gem.platforms correctly, which is what --platform modifies.

After this PR:

$ ruby -Ilib bin/gem install sorbet-static -v 0.5.5460 --platform universal-darwin-19 --backtrace
Successfully installed sorbet-static-0.5.5460-universal-darwin-19
1 gem installed

Closes #4510, the only real issue in RubyGems or Bundler reported there.

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• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
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