[nixpkgs/nix-flakes] add ~/.nix-profile/bin to PATH

Would be great to be able to easily install packages in the docker image (works using nix shell, but that's awkward).

I think the best option would be to just add `~/.nix-profile/bin` to the PATH, wouldn't it?

Allowing:
```
$ nix profile install nixpkgs#rsync
$ rsync --help
```