nixos/installer: force root fs type

installer media can be used on top of existing host configs. In such
scenarions, root fs types will already be defined.

Before this change, this will inevitably lead to the following error:
```console
error: The option `fileSystems./.fsType' has conflicting definition values:
       - In `/nix/store/2nl5cl4mf6vnldpbxhrbzfh0n8rsv9fm-source/DevOS/os/hardware/common.nix': "ext4"
       - In `/nix/store/jbch90yqx6gg1h3fq30jjj2b6h6jfjgs-source/nixos/modules/installer/cd-dvd/iso-image.nix': "tmpfs"
```

With this patch, the installers will override those values according to
their own local requirement.

Use `mkOverride 60` so that conscientious overriding specially targeted
at the installer, e.g. with `mkForce` is still straight forward.

nixos/modules/installer/cd-dvd/iso-image.nix

@@ -654,7 +654,11 @@ in
       ];
 
     fileSystems."/" =
-      { fsType = "tmpfs";
+      # This module is often over-layed onto an existing host config
+      # that defines `/`. We use mkOverride 60 to override standard
+      # values, but at the same time leave room for mkForce values
+      # targeted at the image build.
+      { fsType = mkOverride 60 "tmpfs";
         options = [ "mode=0755" ];
       };
 

nixos/modules/installer/netboot/netboot.nix

@@ -30,7 +30,11 @@ with lib;
           else [ pkgs.grub2 pkgs.syslinux ]);
 
     fileSystems."/" =
-      { fsType = "tmpfs";
+      # This module is often over-layed onto an existing host config
+      # that defines `/`. We use mkOverride 60 to override standard
+      # values, but at the same time leave room for mkForce values
+      # targeted at the image build.
+      { fsType = mkOverride 60 "tmpfs";
         options = [ "mode=0755" ];
       };