Don't overwrite already-copied files on install

Fixes NixOS#6679 and all issues that contain
`cp: cannot overwrite directory ... with non-directory` errors.
These were caused by 475fc10 and
bb0c4b9. Or rather, installations after
475fc10 erroneously followed and deep-copied symlinks, which was fixed
in bb0c4b9. This meant installations installed with the installer
released between these commits had some paths in their nix store with
directories where symlinks should have been, causing the fixed installer
to try to overwrite them with symlinks.

The -n will not overwrite existing files, which is fine inside of the
nix-store as identical store paths will have identical content.

For additional details and examples, see
NixOS#7603 (comment)

scripts/install-multi-user.sh

@@ -816,7 +816,7 @@ install_from_extracted_nix() {
         cd "$EXTRACTED_NIX_PATH"
 
         _sudo "to copy the basic Nix files to the new store at $NIX_ROOT/store" \
-              cp -RPp ./store/* "$NIX_ROOT/store/"
+              cp -RPnp ./store/* "$NIX_ROOT/store/"
 
         _sudo "to make the new store non-writable at $NIX_ROOT/store" \
               chmod -R ugo-w "$NIX_ROOT/store/"