Reconsider setting TAILS_CHANNEL=alpha for alphas/betas/RCs
Since ecfceac5 we started to set
/etc/os-release for alphas/betas/RCs. A
problem with this vs our current release process is that people running
such release will get the updates we publish on the alpha channels (for
our manual test session) before they are released.
Feature Branch: bugfix/12458-channel