selftests/bpf: Use strscpy in bpftool_helpers.c

selftests/bpf: Use strscpy in bpftool_helpers.c

Replace strncpy() calls in bpftool_helpers.c with strscpy().

Pass the destination buffer size to detect_bpftool_path() instead of
hardcoding BPFTOOL_PATH_MAX_LEN.

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://lore.kernel.org/r/20260223190736.649171-5-ihor.solodrai@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>