This makes the code and the tests use the same detection for the channel, to avoid the issue where
ESR builds fail whilst they are setting up the branch (the update channel is set, but the version is not).
Originally, I was thinking that this was reasonable test to ensure the settings are correct on ESR, but it
ends up that the test is only expecting the branch to be on release. Hence, it isn't really a valid test case
and we should trust that the ESR flags are correctly set once ESR starts, and that there's QA/other testing
already in place to verify that fact.
Differential Revision: https://phabricator.services.mozilla.com/D181218
277ae4e069