selftests/bpf: Set test path for token/obj_priv_implicit_token_envvar

commit f01750ae upstream.

token/obj_priv_implicit_token_envvar test may fail in an environment
where the process executing tests can not write to the root path.

Example:
https://github.com/libbpf/libbpf/actions/runs/11844507007/job/33007897936



Change default path used by the test to /tmp/bpf-token-fs, and make it
runtime configurable via an environment variable.

Signed-off-by: Ihor Solodrai <ihor.solodrai@pm.me>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20241115003853.864397-1-ihor.solodrai@pm.me


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>