Daily builds of the Linux kernel with Launchpad

How can I create a PPA with a source package recipe to build a patched Linux kernel that tracks the mainline Ubuntu kernel?

I'm struggling to even figure out which Launchpad project to build from.

lp:linux has only a master branch so I'm not sure how to build the code for a given Ubuntu release. (It seems to be identical to lp:ubuntu/+source/linux, but is it?) Compare to lp:ubuntu/+source/linux-meta which has branches like applied/ubuntu/focal-security.

There is lp:~ubuntu-kernel/ubuntu/+source/linux/+git/focal whose master-prep branch has tags that correspond with the package versions I see in the apt repositories, but I doubt I can construct a source package recipe that looks for new tags. (Also, the current tag is ahead of the latest released package version in the apt repos.)

However, the simplest recipe that uses this branch would be:

# git-build-recipe format 0.4 deb-version {debupstream}-0~{revtime}lp:~ubuntu-kernel/ubuntu/+source/linux/+git/focal master-prep

It fails because the repository doesn't contain a debian/control file:

RUN ['git-build-recipe', '--safe', '--no-build', '--manifest', '/home/buildd/build-RECIPEBRANCHBUILD-3556697/chroot-autobuild/home/buildd/work/tree/manifest', '--distribution', 'focal', '--allow-fallback-to-native', '--append-version', '~ubuntu20.04.1', '/home/buildd/build-RECIPEBRANCHBUILD-3556697/chroot-autobuild/home/buildd/work/recipe', '/home/buildd/build-RECIPEBRANCHBUILD-3556697/chroot-autobuild/home/buildd/work/tree']...git-build-recipe: error: No control file to take the package name from, and --package not specified.

When building manually, according to the Ubuntu wiki, you need to run LANG=C fakeroot debian/rules clean to generate debian/control. But Launchpad won't let me use the run command in my recipe to invoke this.