server: background job inheriting from cluster setting

In https://github.com/cockroachdb/cockroach/pull/101486, we started inheriting from cluster settings. This change makes it so that instead of the go default for session variables, background jobs inherit from the global default (cluster setting's) value. This does not make particular sense in cases like statement timeouts. There may be other instances where we do not prefer global defaults to be applied to background jobs; this issue will track the existence of the pattern.

For a fix/more context, see: https://github.com/cockroachdb/cockroach/pull/126336, https://github.com/cockroachlabs/support/issues/2998

Jira issue: CRDB-40144