CI: GnuTests: Split native and selinux tests to 2 different jobs (#8388)

* .github/workflows/GnuTests.yml: Move variables to env

Will make it easier to split the work into 2 jobs.

* .github/workflows/GnuTests.yml: Move all Selinux steps to a separate job

* .github/workflows/GnuTests.yml: Create a new summary job

That aggregates between native and selinux results.

* .github/workflows/GnuTests.yml: Fix log uploading

This... didn't work, even in the previous version.

(found by Gemini...)

* .github/workflows/GnuTests.yml: Improve aggregate

Variables don't always help make things clearer.

* .github/workflows/GnuTests.yml: Make selinux and native flow identical

Also add some headers to make things easier to follow.

* .github/workflows/GnuTests.yml: More misc cleanup

In particular, SELinux flow is simplified, and fixed.

Also fix artifact download name.

* .github/workflows/GnuTests.yml: Fix aggregate job

Author: drinkcat