sql: override StmtTimeout to 0 for background jobs

For background jobs and schema changes, our Internal
Executor inherits from cluster settings. If a statement
timeout is set cluster-wide, then background jobs will
respect this -- which does not make much sense; thus,
this patch overrides the StmtTimeout to 0s for background
jobs.

Fixes: #126261
Release note (bug fix): Background jobs no longer respect
a statement timeout.