This change updates the CI build workflow to only trigger on pull
requests to the master branch. The release branches almost never cause
build errors, so we can skip that check.
This change updates the build pipeline to build the kernel with the
default GCC version on Debian Buster. This is more safe for people
running custom kernel modules.
This change updates the build code to support in-tree builds. This is
easier to manage for CI builds and reduces the amount of IO operations
necessary for the build process.
