ci: add workflow to clean caches periodically

[8144225309]

Deletes all caches on the 1st and 15th of each month.

Fixes #1691

[real-or-random]

Concept ACK

I'm a bit hesitant due to the actions: write permission, but I tend to think it's okay here. This workflow doesn't even have an input that could be attacker controlled. And actions restricts it to GitHub Actions stuff.

[real-or-random]

ACK a27ae1a

[real-or-random]

cc @hebasto

[hebasto]

It's shame that https://github.com/actions/cache does not provide an option to set a retention period explicitly.

I think we could avoid introducing a new workflow with the write permission by simply rotating the cache keys regularly. Feel free to grab a commit from this branch.

[real-or-random]

I think we could avoid introducing a new workflow with the write permission by simply rotating the cache keys regularly.

That seems cleaner indeed.

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edit: This means for this PR:

Approach NACK

[hebasto]

Unrelated to the changes in this PR, the current GCC snapshot has been failing self-tests since ed2924c7628343ff8b699f320a05c85bdb9c2837.

UPDATE: The issue should be fixed in a6c3c48148dd60a09ffab57019f163180c159d65.