Run a periodic esmvalbot test via automated PR

[valeriupredoi]

Description

This PR adds a Github Action that sets up an automated PR (via an empty commit, so no actual files changed, even if that PR gets merged by accident) where Esmvalbot gets invoked to run an example recipe (recipe_python.yml, but that can change) via comment from the GH bot, to test our bot periodically. This is how the auto PR looks like #3468 (don't mind the failed codacy, the auto PR will target main after this PR gets merged). The only hitch at the moment is to allow github-actions[bot] to invoke Esmvalbot, easily done if peeps are OK with it - the rest is as if it was a use ie open PR, invoking Esmvalbot via comment, labels etc that reminds @valeriupredoi @bouweandela and @schlunma to run a Esmvalbot test, see #3804 - previously the github-actions bot was invoking ESMValBot but that's not possible unless the GH bot is added as a user in esmvalbot's books - and that's a bit contentious and probably not safe - now all it does it pings us to run a test, every 10 days or so, but that can change to every month or so.

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[bouweandela]

The only hitch at the moment is to allow github-actions[bot] to invoke Esmvalbot, easily done if peeps are OK with it

I'm not sure if we would be allowed to do that from a security point of view. I also do not feel qualified to make a judgment on that. Is this an important topic to you? Maybe we could discuss it with the providers of the machine that the bot runs on.

[mo-gill]

Hi, we are currently working on the ESMValTool release for v2.11.0. We're wondering if you'd be able to complete this PR by the end of next week (Friday 10th May).

Otherwise, please let us know, and we'll move it into the next milestone for you 🙂

[valeriupredoi]

am moving this to M2.12 since we're still not decided if the bot can have that much powah 🤖

[valeriupredoi]

The only hitch at the moment is to allow github-actions[bot] to invoke Esmvalbot, easily done if peeps are OK with it

I'm not sure if we would be allowed to do that from a security point of view. I also do not feel qualified to make a judgment on that. Is this an important topic to you? Maybe we could discuss it with the providers of the machine that the bot runs on.

converted this to a simple "reminder to check the bot" PR @bouweandela - a lot safer and less fiddly 🍺