Reconsider setting TAILS_CHANNEL=alpha for alphas/betas/RCs

Originally created by @anonym on #12458 (Redmine)

Since ecfceac5 we started to set TAILS_CHANNEL=alpha in /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

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