build(single_file_libs): don't close sys.stdout in combine.py

[vidigoat]

combine.py's output handle destn defaults to sys.stdout and is only reassigned to a real file when -o/--output is supplied. The finally block then calls destn.close() unconditionally — so running without -o (a documented mode; the -o help says output goes to stdout otherwise) closes sys.stdout.

Fix: only close the handle when it isn't sys.stdout.

Verified locally: with no -o, the stdout stream is no longer left closed after the run; with -o, the output file is still written/closed as before.

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