The problem is seen on CI, where TEMP pointed to directory outside of
the usual vardir, when testing mysql_install_db.exe
A likely cause for this error is that TEMP was periodically cleaned up
by some automation running on the host, perhaps by buildbot itself.

To fix, mysql_install_db.exe will now use datadir as --tmpdir
for the bootstrap run. This will minimize chances to run into any
environment problems.