ci: add a new workflow to test bpftool build

[Tropicao]

The tools/testing/selftests/bpf directory in the kernel source tree contains the test_bpftool_build.sh script that ensures that the various supported ways of building bpftool work correctly. This test isn't currently part of the tests automatically executed in CI.

Add a new layer of workflows in test.yml to run tests that will exercise build commands. As a first sub-workflow, add a bpftool build workflow:
test.yml -> test-build-commands.yml -> build->bpftool.yml
This test-build-commands workflow can be extended later to support other artifacts build test, for example to test the different ways of building selftests.

The new build-bpftool workflow aims to validate the different ways of building the bpftool CLI, as exercised by the test_bpftool_build.sh in the kernel source tree (in tools/testing/selftests/bpf). As the test has only a few dependencies (only a build test, no dependency on built artifact), the added workflow remains simple and isolated from the other "build-and-tests" workflows:

• it only tests host build, so no need to run it as many time as the matrix script generates configurations
• supports download_sources flag, so can be run from kernel tree or vmtest tree

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This new workflow does not make the whole test_bpftool_build.sh execute in CI; as some subtests are gated by the presence of a .config. I suspect this to be hiding a broader kbuild issue as bpftool does not really depend on .config, I'll fix this separately in another series.

[theihor]

@Tropicao the CI run with this change hasn't even started.

The problem is probably with permissions: read-all: the workflow that is called inherits permissions of the caller, and can not expand them (and in this case read-all does).

[theihor]

Please respin.