selftests/bpf: Choose pkg-config for the target

pkg-config is used to build sign-file executable. It should use the
library for the target instead of the host as it is called during tests.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20231125084253.85025-2-akihiko.odaki@daynix.com