machines: moby: enable impermanence (experimental)

last time i tried to rebuild nixos on the pinephone it didn't switch,
IIRC. i don't know why, but i expect it'll be easier to manage my
machines if i keep more of the setup consistent across them.

likely i'll tune this to be more leniant on moby (e.g. persist all of
.librewolf).

machines/moby/default.nix

@@ -32,6 +32,7 @@
   ];
 
   sane.nixcache.enable = true;
+  sane.impermanence.enable = true;  # XXX experimental
   sane.gui.phosh.enable = true;
   boot.loader.grub.enable = false;
   mobile.bootloader.enable = false;