Poor Github Actions support

[guijan]

It would be nice if termux-docker supported Github Actions better. Hopefully, people will add Termux to their CI.

I made a test repository to showcase it: guijan/termux-on-gha

The issue is that right now there's no documentation on how to get it working, and the straightforward way is broken. Here's the current state of things:
https://github.com/guijan/termux-on-gha/blob/a9a2cf26be0299c1643be12acfa301b0465ad2c1/.github/workflows/build-and-test.yml
The file is short and commented, hopefully it's easy to follow.

Users need to prefix pkg and pip with the /entrypoint.sh script because otherwise they run as the root user and error. This happens because github overrides the entrypoint, as can be seen here: https://github.com/orgs/community/discussions/26465

If you fix the issue above, you will run into further breakage. The widely used checkout and upload-artifact actions don't currently work with Termux. The checkout action errors because of the same problem as this issue I found out there (no /etc/os-release file, and possibly more): prometheus/prometheus#12656
Someone has suggested adding an /etc/os-release to Termux before: termux/termux-app#2620

I haven't investigated why the upload-artifact action fails.