_srt_get_helpers_path: If in /lib, assume helpers are in /usr/libexec
On a system with merged-/usr, steam-runtime-tools might be installed in /usr/lib/MULTIARCH (or /usr/lib64 or /lib64) but found via the /lib -> usr/lib or /lib64 -> usr/lib64 symlink. /libexec doesn't normally exist, so look for the helpers in /usr/libexec instead.
Found by !10 (merged).
/cc @denittis @jpwhiting