fix: prevent duplicate process.exit() on concurrent unhandledRejections

[Gopi-Agasthia-S-005CIU744]

When multiple package downloads time out simultaneously (e.g. from a private
Artifactory registry), Node.js fires multiple unhandledRejection events in
quick succession. Each one enters #handleExit, which defers process.exit()
via a stderr.write → stdout.write callback chain.

Without a re-entrancy guard, both deferred callbacks complete independently:

1. First rejection sets #exited = true, queues a deferred process.exit().
2. Second rejection enters #handleExit before any I/O callback has run,
   sets #exited = true again, queues a second deferred process.exit().
3. First process.exit() fires → emits 'exit' → #handleProcessExitAndReset
   resets #exited back to false.
4. Second process.exit() fires → emits 'exit' → #handleProcessExit sees
   #exited === false → logs "Exit handler never called!".

Fix adds a two-line re-entrancy guard at the top of #handleExit so only the
first rejection ever schedules a process.exit().

The regression test defers stderr.write via setImmediate to open the same
race window that real I/O creates in production, fires two unhandledRejection
events synchronously, and asserts process.exit() was called exactly once.
The test fails without the fix and passes with it.

References

Fixes #9739