Fix crash on Linux when testing the archive plugin

[sebsto]

Fix a rare crash in CI on the archive plugin.

The crash is a race condition in PluginUtils.swift's execute method. The code set a terminationHandler on the Process that called readToEnd() on the pipe, while simultaneously the readabilityHandler could still be firing on the same pipe's file handle. On Linux (x86_64, Ubuntu 24.04 in CI), this race corrupts memory during Swift runtime metadata resolution (swift_conformsToProtocol, _swift_getGenericMetadata), which manifests as the SIGSEGV we're seeing in _dispatch_event_loop_drain.

The fix:

I removed the terminationHandler entirely. Since waitUntilExit() is already called synchronously, we know the process is done.
After waitUntilExit(), we set readabilityHandler = nil to stop the async reads, then do one final readToEnd() on the output queue to drain any remaining data.
This eliminates the race between the readability handler and the termination handler competing over the same file handle.