windows: Don't try to breakaway from an existing Job in batch mode.

Reasoning: It never worked anyway, because Bazel didn't set the
JOB_OBJECT_LIMIT_BREAKAWAY_OK limit on its job. This is why running
Bazel in batch mode inside a shell integration test on Windows caused a
"CreateProcess() failed: access denied" error.

By no longer using CREATE_BREAKAWAY_FROM_JOB, this will instead cause us
to use nested jobs, which is exactly the right thing to do:
- If our parent dies, we and our children still get reliably killed.
- But if *only* we die, then just our children will be reliably killed.

If we're on an older Windows that doesn't support nested jobs and are
running inside an existing job, we should assume that our parent handles
process management - for example like Bazel, which runs all spawned
processes in their own job.

Also remove the CREATE_NEW_PROCESS_GROUP flag in Bazel's batch mode,
because I can't see how this makes sense for a non-daemon process.

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PiperOrigin-RevId: 150194586
MOS_MIGRATED_REVID=150194586

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