Using -n for same-host SSH operations is not POSIX compliant

[bluekeyes]

https://github.com/palantir/giraffe/blob/03182397d7a0476df192094f2730ea169b4979e8/ssh/src/main/java/com/palantir/giraffe/ssh/internal/SshSameHostFileHelper.java#L33

https://github.com/palantir/giraffe/blob/03182397d7a0476df192094f2730ea169b4979e8/ssh/src/main/java/com/palantir/giraffe/ssh/internal/SshSameHostFileHelper.java#L63

The -n flag (to prevent overwriting existing files) is not part of the POSIX specification for the cp or mv commands. Giraffe guarantees that it will work with any POSIX-compliant shell, so this is a violation of the contract. That said, I'm not sure how to fix this and it hasn't caused problems for any real users yet. I just ran into while updating the CI tests to an Alpine container.