unsafe caching of .fmt files

[Han-Wen Nienhuys]

Hi there,

while building the LilyPond regression test in clean container with
parallelism, I am seeing file corruption errors from reading back TeX
.fmt files.

AFAICT, the various flavors of TeX cache .fmt files in ~/.texlive2019.
This file is re-created when it doesn't exist, but no care is taken
multiple parallel invocations of the same program do not tread on each
others feet. Either some invocations see a half-finished version of
the file, or two invocations write content to the same file.

-- 
Han-Wen Nienhuys - hanwenn at gmail.com - http://www.xs4all.nl/~hanwen