JSON for Hash now has to be Base16

Fix #14532.

As discussed on the call today:

1. We'll stick with `format = "base16"` and `hash = "<hash>"`, not do
   `base16 = "<hash>"`, in order to be forward compatible with
   supporting more versioning formats.

   The motivation we discussed for someday *possibly* doing this is
   making it easier to write very slap-dash lang2nix tools that create
   (not consume) derivations with dynamic derivations.

2. We will remove support for non-base16 (and make that the default, not
   base64) in `Hash`, so this is strictly forward contingency, *not*
   yet something we support. (And also not something we have concrete
   plans to start supporting.)

tests/functional/fixed.sh

@@ -15,13 +15,12 @@ nix-build fixed.nix -A bad --no-out-link && fail "should fail"
 # a side-effect.
 [[ -e $path ]]
 nix path-info --json "$path" | jq -e \
-    --arg hash "$(nix hash convert --to base64 "md5:8ddd8be4b179a529afa5f2ffae4b9858")" \
     '.[].ca == {
         method: "flat",
         hash: {
             algorithm: "md5",
-            format: "base64",
-            hash: $hash
+            format: "base16",
+            hash: "8ddd8be4b179a529afa5f2ffae4b9858"
         },
     }'