feat: update image label with actual kernel version

[bsherman]

Since we use differing kernels from upstream, we should be properly setting the ostree.linux label on the image to reflect the actual kernel within the image.

Relates: ublue-os/hwe#220

[bsherman]

As I have a PR which doesn't work, I'm adding a thought here to provide context:

I expect we can do this by running a "podman run/exec" in the "Get current version" step, and within that podman container environment, use dnf to query what kernel WOULD be installed given the build-args and env vars for the kernel flavor. Then echo that kernel version out ... eg

echo "SOURCE_IMAGE_LINUX=$linux" >> $GITHUB_ENV