[tlbuild] luaharfbuzz breaks tostring

[Akira Kakuto]

Dear Simon,

> If I iterate over luaharfbuzz in texlua it dies with
> a broken tostring()
> ...

We recovered texlua[jit][c] as symbolic links to lua[jit]tex,
because they, as symbolic links to lua[jit]hbtex, broke context
and contextjit.

If you want luaharfbuzz in lua mode, please use
'luahbtex --luaonly' instead of 'texlua'.

Thanks,
Akira