MFS r360795: lualoader: fix loader.lua mis-merge

This mis-merge came from r360599; r331321 introduced core.isUEFIBoot but
will not get merged back to stable/11 due to being somewhat disruptive. I
had resolved the conflict once, but had to re-do the merge locally due to
another conflict and failed to resolve it the second time.

This mistake is explosive to those trying to run lualoader on stable/11, but
is mitigated by the fact that lualoader is not the default here.

PR: 246291
Approved by: re (gjb)