runners: Add option to install custom kernel on Fedora

Allow installing a custom kernel version from the Fedora experimental
kernel repos onto the github runners. This is useful for testing if
ZFS works against a newer kernel.

Fedora has a number of repos with experimental kernel packages. This
PR allows installs from kernels in these repos:

@kernel-vanilla/stable
@kernel-vanilla/mainline
(https://fedoraproject.org/wiki/Kernel_Vanilla_Repositories)

You will need to manually kick of a github runner to test with a custom
kernel version. To do that, go to the github actions tab under
'zfs-qemu' and click the drop-down for 'run workflow'. In there you
will see a text box to specify the version (like '6.14'). The scripts
will do their best to match the version to the newest matching version
that the repos support (since they're may be multiple nightly versions
of, say, '6.14'). A full list of kernel versions can be seen in the
dependency stage output if you kick off a manual run.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #17156