feat(justfile): add overlay recipe with sysext for testing

[tulilirockz]

This will allow us to test packages easier and in a persistent manner via systemd-sysext by automatically creating layers based on the selected RPMs. This REQUIRES systemd 257 as before that we didnt get the selinux patch for sysext. This sets selinux contexts automatically for the RPMs too so they should be able to mount/run just fine by default

The expected workflow is:

• just build (specfile)
• just overlay ./mock/(chroot)/results/(my rpm).rpm

[bsherman]

Looks very useful and it helps encourage testing on a running system.

Granted it's best used if someone is running Bluefin LTS, since otherwise the test is of questionable value.