Update a leftover doc for unpackexe

[muzimuzhi]

It defaults to pdftex since release 2019-09-14,
commt 1c6879d (Use pdftex for unpacking, 2019-09-14)

[muzimuzhi]

CI failed right after platex.fmt is built. Digging

Transcript written on platex.log.
mktexfmt [INFO]: log file copied to: /home/runner/texlive/texmf-var/web2c/euptex/platex.log
mktexfmt [INFO]: /home/runner/texlive/texmf-var/web2c/euptex/platex.fmt installed.
mktexfmt [INFO]: successfully rebuilt formats: 1
mktexfmt [INFO]: not selected formats: 20
mktexfmt [INFO]: total formats: 21
mktexfmt [INFO]: exiting with status 0
./l3build-check.lua:94: ./build/test/00-test-1.log: No such file or directory

====================
Tests failed, zip stage skipped!
====================

https://github.com/latex3/l3build/actions/runs/5568597782/jobs/10171348277#step:5:200

[muzimuzhi]

platex.fmt should be built at step "Install TeX Live", but now it's built at step "Run l3build".

Update: Before, platex.fmt/eptex is built, but now platex.fmt/euptex is built. And then it's rebuilt

mktexfmt [INFO]: writing formats under /home/runner/texlive/texmf-var/web2c
mktexfmt [INFO]: --- remaking platex with euptex

[josephwright]

I'll merge then worry about GH Actions!