Avoid using environment variables in a constant

The following discussion from !2396 (merged) should be addressed:

• @godfat-gitlab started a discussion: (+1 comment)

  This seems to break my local tests. I am not sure why it's working on CI 🤔 The error I am getting is:

  Failure/Error: TOKEN_HEADER = { headers: { 'Private-Token': Triage.api_token(:com) } }.freeze
  
  KeyError:
    key not found: "GITLAB_COM_API_TOKEN"

  This is expected, because this is outside of RSpec and we only stub the token inside RSpec.