also compile Lmod spider cache for alternate Lua versions if corresponding luac command is available

[boegel]

This is useful when for example /usr/bin/luac-5.3 is available on the host where the spider cache is generated, while /usr/bin/luac is Lua 5.1.

That way we can generate a compiled spider cache for both CentOS7 (Lua 5.1) and RHEL8 (Lua 5.3) on a single system and in one go.