runners: Add option to specify Ubuntu nightly kernel

[tonyhutter]

Allow installing a custom kernel from the Ubuntu nightly packages page to test ZFS against new kernels. Specifically, add a text box to specify the URL you want the runner to scrape and install from. For example:

https://kernel.ubuntu.com/mainline/v6.14-rc7/

This option only works on workflow_dispatch, so the user will need to enter in the URL on the actions page and run the workflow manually.

Motivation and Context

Description

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.