os: test that copying to append-only files doesn't fail on Linux

os: test that copying to append-only files doesn't fail on Linux

Before CL 494915, this test would fail on Linux in io.Copy with error
"write /dev/stdout: copy_file_range: bad file descriptor" because the
copy_file_range syscall doesn't support destination files opened with
O_APPEND, see
https://man7.org/linux/man-pages/man2/copy_file_range.2.html#ERRORS

For #60181

Change-Id: I2eb4bcac71175121821e0033eb2297a2bc4ec759
Reviewed-on: https://go-review.googlesource.com/c/go/+/517755
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>