CreateOrMergeNixConfig: set expected nix.conf mode to 644, not 664

It has apparently been like this since the start[1], but that was
probably a typo. Let's stop perpetuating it.

[1]: https://github.com/NixOS/nix/blob/121020fc508b5731209cc158986f1e78fb99ed9f/scripts/install-multi-user.sh#L967

src/action/base/create_or_merge_nix_config.rs

@@ -18,7 +18,7 @@ use crate::action::{
 /// The `nix.conf` configuration names that are safe to merge.
 // FIXME(@cole-h): make configurable by downstream users?
 const MERGEABLE_CONF_NAMES: &[&str] = &["experimental-features"];
-const NIX_CONF_MODE: u32 = 0o664;
+const NIX_CONF_MODE: u32 = 0o644;
 const NIX_CONF_COMMENT_CHAR: char = '#';
 
 #[non_exhaustive]