[tlbuild] luaharfbuzz breaks tostring

[Akira Kakuto]

Dear Simon,

> It also breaks in "luahbtex --luaonly".

I didn't say that "luahbtex --luaonly" did not break it.
In the early pretest of TL2020, texlua was a symbolic link
to luahbtex. So in the early pretest, "texlua" and "luahbtex --luaonly"
were equivalent.
However in the latest pretest, texlua becomes a
symbolic link to luatex, which is not linked with luaharfbuzz.
Thus your example itself does not work by "texlua ..." in the
latest pretest. You have to execute "luahbtex --luaonly ..." instead.

Thanks,
Akira