selftests/bpf: Override PKG_CONFIG for static builds

A library may need to depend on additional archive files for static
builds so pkg-config should be instructed to list them.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20231125084253.85025-3-akihiko.odaki@daynix.com