pkgbase: switch to a PATH search for flua

[kevans]

Summary

Similar to how the top-level make sysent does it, inject libexec into
$PATH and search it for flua instead of relying on a hardcoded flua.

This gives us the flexibility to use a bootstrap flua if it has been
built instead of the system flua while still using the system flua where
it's been deemed adequate.

[ivy]

not tested but this looks fine in principle