os: don't try to put directory into non-blocking mode

Fixes #56843

Change-Id: I3cb3e8397499cd8c57a3edddd45f38c510519b36
Reviewed-on: https://go-review.googlesource.com/c/go/+/451997


Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Nicolas Hillegeer <aktau@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Rob Pike <r@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
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