MDEV-34428 bootstrap can't delete tempfile, it is already gone

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.

sql/mysql_install_db.cc

@@ -333,6 +333,7 @@ static char *init_bootstrap_command_line(char *cmdline, size_t size)
     " %s"
     " --bootstrap"
     " --datadir=."
+    " --tmpdir=."
     " --loose-innodb-buffer-pool-size=20M"
     "\""
     , mysqld_path, opt_verbose_bootstrap ? "--console" : "");