Ensure 3.0~beta1 can be incrementally upgraded to 3.0~beta2, etc.
I think the nicest way to do that is to configure
/etc/os-release so that the alpha update channel is used. And we'll revert that in 3.0 final.
To avoid breaking the test suite, an UDF for 1.0~test / amd64 should be added to the alpha channel.
Upgrader: use the alpha channel when the next version will be an alpha, beta, or RC.
The basic idea here is to make it easy for users of 3.0~betaN to upgrade to the
next beta or RC, without having to type any command-line, and without having to
create UDFs for these upgrade paths on the "stable" update channel (that would
feel wrong, wouldn't it?).
Bonus: once this is in place, more generally we'll be able to update our release
process doc+scripts to stop creating meaningless "stable" channel upgrades for
RCs, such as upgrade/v1/Tails/2.10~rc1/i386/stable/upgrades.yml, that we have
been needing so far since our RCs thought they were using the "stable"