The execution of mtr in the Windows environment fails due to
the fact that the new code from MDEV-18565 does not take into
account the need to add the ".exe" extension to the names of
executable files when searching for pre-requisites that are
needed to run SST scripts (especially when using mariabackup)
and when searching paths to some other Galera utilities.
This patch fixes this flaw.

Also adding paths to the PATH environment variable is now
done with the correct delimiter character.