Refactor the QA framework to make Runtime::Env read-only

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The following discussion from gitlab-ce!29937 should be addressed:

• @mlapierre started a discussion: (+4 comments)

  I don't think we should change the value of anything in Runtime::Env -- it's supposed to represent environment variables, so it should be read-only.

This caused problems previously because we assume Runtime::Env will always contain the values set in the environment, so when a test changed Runtime::Env.personal_access_token subsequent tests failed.