Golang os: CopyFS overwrites existing file in destination. [1.23 backport]

@panjf2000 requested issue #68895 to be considered for backport to the next 1.23 minor release.

@gopherbot please create a backport to 1.23, it's a mismatch of doc and actual behavior.

Comment From: gopherbot

Change https://go.dev/cl/606415 mentions this issue: [release-branch.go1.23] os: use O_EXCL instead of O_TRUNC in CopyFS to disallow rewriting existing files does not exist

Comment From: gopherbot

Closed by merging CL 606415 (commit 3c9340557cf1e13c2fe61c6b894ba284711dfb85) to release-branch.go1.23.