nixos/filesystems: Remove default = "auto" from fsType

NixOS has traditionally enabled the `ext` family of file systems by
default. Originally, when switching to systemd initrd, we wanted to
transition to making this explicit so that initrds could be made
without `ext`. The problem is that anyone with `fsType = "auto";` for
an `ext` file system in initrd will fail to boot, which is not really
an acceptable regression as we switch to systemd initrd by default.

By removing `default = "auto"` from `fsType`, we rule out the vast
majority of these regressions as eval errors, since most users of
`fsType = "auto"` for ext file systems are using it because of the
default value.

In hindsight, this is probably what #225352 was really about.

flake.nix

@@ -144,6 +144,7 @@
                         nixpkgs.hostPlatform = "x86_64-linux";
                         boot.loader.grub.enable = false;
                         fileSystems."/".device = "nodev";
+                        fileSystems."/".fsType = "none";
                         # See https://search.nixos.org/options?show=system.stateVersion&query=stateversion
                         system.stateVersion = lib.trivial.release; # DON'T do this in real configs!
                       }