This is necessary to support postgres 11 in Debian Buster.
This lets us leave build-pgroonga platform-agnostic.
/bin/sh and /usr/bin/env are the only two binaries that NixOS provides at a fixed path (outside a buildFHSUserEnv sandbox). This discussion was split from #11004. Signed-off-by: Anders Kaseorg <andersk@mit.edu>