.github/workflows/push.yml: add job to build out-of-tree extensions

[MaxKellermann]

This aims to detect API breakages early by compiling a selection of out-of-tree extensions in the CI.

[MaxKellermann]

This was surprisingly easy - right now, it builds APCu and xdebug. Suggestions for more extensions welcome (now or after this is merged).
Note that this PR contains #10395 because it also uses ccache, and not doing so would not make sense. As soon as ccache is merged, I'll rebase this PR and un-"draft" it.

[MaxKellermann]

Added a few more extensions from the PECL top 10 (https://pecl.php.net/package-stats.php).
I'll simplify the workflow by moving the checkout/build step into a separate GH action, and calling this for every extension.

[iluuu1994]

@MaxKellermann Thank you! This makes sense to me but should be moved to nightly. I'm also not sure if a failing build might be annoying as it is unavoidable for master, especially if we can't fix the 3rd party code.

[MaxKellermann]

This makes sense to me but should be moved to nightly.

I can easily move it to nighly.yml as-is, and it will work just as well, I guess, but I guess I can't test it then?
I suggest I do that as soon as the rest is considered "done".

I'm also not sure if a failing build might be annoying as it is unavoidable for master, especially if we can't fix the 3rd party code.

Do you mean php-src master or extension's master? We could easily build specific versions of those extensions instead of their master branch, and probably should. Only the latest release should be considered stable enough. Of course, there will be breakages eventually, and that will be annoying, but it's a first step towards fixing those.
Once an extension breaks with no hope to get it fixed, it should be dropped from PHP's CI, of course.

[iluuu1994]

I suggest I do that as soon as the rest is considered "done".

That's what I've usually done.

Do you mean php-src master or extension's master?

I meant php-src's master, given that most API breaks there are intentional. Your suggestion sounds good to me.

[MaxKellermann]

@iluuu1994 I've moved the rules to nightly.yml as you suggested.
Please consider merging this now.

[iluuu1994]

Thank you @MaxKellermann!