[Buildbot][z/OS] Disable threads for z/OS builder and reduce jobs for worker (#926)

This PR disables threads for the z/OS builder `llvm-s390x-zos` and
reduces the jobs from 4 to 1 for the `zos-s390x-1` worker.

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Co-authored-by: Raihaan Sandhu <raihaansandhu@dhcp-9-95-182-119.4em-ca.ibm.com>