Migrate i386 to GitHub actions

[iluuu1994]

No description provided.

[iluuu1994]

This executes ~100 tests less, most because it runs as root. Running the container with a different user doesn't seem possible on GitHub actions (https://docs.github.com/en/actions/creating-actions/dockerfile-support-for-github-actions#user) (unless I'm misinterpreting this statement).

[cmb69]

This skips the test on Windows, although it otherwise would work fine. On Windows, there is no need to check the euid at all, which could easily be fixed. However, the follow-up fix (d2c6634) which is fine per se, makes it more difficult to also run the test on Windows (due to having posix in the --EXTENSION-- section). Maybe we should split this test. Or, if we expect more such posix_geteuid() == 0 (should probably better be posix_geteuid() === 0) checks, then it may make sense to come up with a more general solution (possibly a new marker section which does this check and the skipping behind the scenes). @iluuu1994, thoughts?

[iluuu1994]

I didn't think about Windows. TBH I didn't look into why exactly the test was failing with root, but for this test it's probably fine to remove posix from EXTENSIONS and skip the root check for Windows.