os: use O_EXCL instead of O_TRUNC in CopyFS to disallow rewriting existing files

On Linux, a call to creat() is equivalent to calling open() with flags
equal to O_CREAT|O_WRONLY|O_TRUNC, which applies to other platforms
as well in a similar manner. Thus, to force CopyFS's behavior to
comply with the function comment, we need to replace O_TRUNC with O_EXCL.

Fixes #68895

Change-Id: I3e2ab153609d3c8cf20ce5969d6f3ef593833cd1
Reviewed-on: https://go-review.googlesource.com/c/go/+/606095


Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>