samples: bpf: don't run probes at the local make stage

samples: bpf: don't run probes at the local make stage

Quentin reports that commit a79e41faf4e3 ("samples: bpf: print
a warning about headers_install") is producing the false
positive when make is invoked locally, from the samples/bpf/
directory.

When make is run locally it hits the "all" target, which
will recursively invoke make through the full build system.

Speed up the "local" run which doesn't actually build anything,
and avoid false positives by skipping all the probes if not in
kbuild environment (cover both the new warning and the BTF
probes).

Reported-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>