MDEV-22349 MTR re-bootstrap modifies environment variable MYSQLD_BOOTSTRAP_CMD

mysql-test/mysql-test-run.pl

@@ -3167,7 +3167,8 @@ sub mysql_install_db {
   # ----------------------------------------------------------------------
   # export MYSQLD_BOOTSTRAP_CMD variable containing <path>/mysqld <args>
   # ----------------------------------------------------------------------
-  $ENV{'MYSQLD_BOOTSTRAP_CMD'}= "$exe_mysqld_bootstrap " . join(" ", @$args);
+  $ENV{'MYSQLD_BOOTSTRAP_CMD'}= "$exe_mysqld_bootstrap " . join(" ", @$args)
+    unless defined $ENV{'MYSQLD_BOOTSTRAP_CMD'};
 
   # Extra options can come not only from the command line, but also
   # from option files or combinations. We want them on a command line