modules/persist: change default mounting method to symlink

this changes the plaintext and cryptClearOnBoot stores: private was already symlink-based.
this isn't strictly necessary: the rationale is:
1. `mount` syscall *requires* CAP_SYS_ADMIN (i.e. superuser/suid).
   that's causing problems with sandboxing, particularly ~/private.
   that doesn't affect other stores *yet*, but it may in the future.
2. visibility. i.e. it makes *clear* where anything is persisted.
   if `realpath` doesn't evaluate to `/nix/persist`, then it's not
   persisted.

hosts/by-name/servo/fs.nix

@@ -50,6 +50,7 @@
   sane.persist.stores."ext" = {
     origin = "/mnt/pool/persist";
     storeDescription = "external HDD storage";
+    defaultMethod = "bind";  #< TODO: change to "symlink"?
   };
 
   # increase /tmp space (defaults to 50% of RAM) for building large nix things.

modules/persist/default.nix

@@ -36,7 +36,7 @@ let
       };
       defaultMethod = mkOption {
         type = types.enum [ "bind" "symlink" ];
-        default = "bind";
+        default = "symlink";
         description = ''
           preferred way to link items from the store into the fs
         '';