Add build-scx-selftests action

[theihor]

Add build-scx-selftests action. This action will build tools/testsing/selftests/sched_ext binaries, given kbuild-output and toolchain parameters passed to the action.

The action and its build script are very similar to build-selftests. Build scipt is simplified somewhat.

Action inputs are different:

• repo-root is a new required parameter, because we don't want to rely on the assuption that $REPO_ROOT == github.workspace
• kernel is removed: it controled the location of vmlinux.h, but it is generated from the $KBUILD_OUTPUT/vmlinux, and I don't think we need to be able to customize it (at least not yet)
  • https://github.com/kernel-patches/bpf/blob/bpf-next_base/tools/testing/selftests/sched_ext/Makefile#L125-L131
  • https://github.com/theihor/libbpf-ci/blob/build-scx/build-scx-selftests/build.sh#L30

[theihor]

WIP vmtest change: kernel-patches/vmtest#294

[chantra]

LGTM. Can you add it as a step in

ci/.github/workflows/kernel-build.yml

Lines 108 to 120 in f605c62

	- name: Build selftests
	uses: ./build-selftests
	with:
	arch: ${{ inputs.arch }}
	toolchain: ${{ inputs.toolchain }}
	kbuild-output: ${{ env.KBUILD_OUTPUT }}
	max-make-jobs: 32
	llvm-version: ${{ inputs.llvm-version }}
	env:
	# RELEASE= disables all optimizaions
	# RELEASE=0 adds -O0 make flag
	# RELEASE=1 adds -O2 make flag
	RELEASE: ${{ inputs.release && '1' || '' }}

so we can make sure we are building/packing the content successfully.