Don't copy permission bits when copying files

[kuviman]

I was using NixOS and copying files from nix store resulted in them not having write permissions, so only the first build could succeed, and later ones would fail to overwrite the file. This fix has solved the issue for me.

[MarijnS95]

A similar albeit different thing is done in #21, can you communicate with @Philipp-M what hacks are needed to support nix?

[kuviman]

I guess I failed to find existing PR when creating mine. I think using std::fs::copy is better since it uses some platform apis, and only falls back to io::copy if that's unavailable so I think the other PR is better