MDEV-28782: modify mariadb-tzinfo-to-sql to set 'wsrep*' variables appropriately in cases where Galera is not compiled in

In 3b662c6e, it was discovered that the
values of the 'wsrep_is_on' and 'wsrep_cannot_replicate_tz' variables need
to be overridden for embedded builds to pass

However, there are other build configurations where these variables also
have NULL values.  The mariadb-tzinfo-to-sql script (implemented in
sql/tztime.cc) can be slightly modified to set its 'wsrep_is_on' and
'wsrep_cannot_replicate_tz' variables more predictably in all such cases,
thus allowing the mysql_tzinfo_to_sql_symlink.test test to pass without
any special-casing for particular build types.

See comments:

- https://github.com/MariaDB/server/commit/3b662c6ebd26b54ce534d9e7451cdc31e6c0046c#r78994411
- https://jira.mariadb.org/browse/MDEV-28782?focusedCommentId=230038&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-230038

All new code of the whole pull request, including one or several files that
are either new files or modified ones, are contributed under the BSD-new
license.  I am contributing on behalf of my employer Amazon Web Services,
Inc.