Increase release cadence of alpha builds

https://github.com/ankidroid/Anki-Android/compare/v2.24.0alpha2...v2.24.0alpha3 has 100+ commits, which made reasoning about the cause of a bug diffiicult.

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> On that release cadence front... we should release an alpha every N days/commits [due to above issue diagnostics being difficult]

-- David

https://discord.com/channels/368267295601983490/701922522836369498/1462818106518016094

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> maybe I could -
>
> 1- add a schedule to publish on main
> 2- include a step that only executes if it's a `schedule` workflow event
> 3- have that step check to see when last alpha was, and outputs an "overdue" signal if it was more than N days (suggest N is like...3 - would exhaust 2-digit version space in 297 days, seems fine?)
> 4- rest of workflow altered to respond correctly to "this is schedule and overdue" vs "this is scheduled and not overdue" vs "this is manual publish no matter what"
> because if I think we should release from main pretty frequently (I *auto-publish on merge* in some of my repos 🦹‍♂️ ) then shipping alphas every 3 days seems hard to argue against

-- Mike

https://discord.com/channels/368267295601983490/701922522836369498/1462820331201761416