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/git-hashing/simple-common.sh

@@ -49,12 +49,12 @@ try2 () {
 
     nix path-info --json "$path" | jq -e \
         --arg algo "$hashAlgo" \
-        --arg hash "$(nix hash convert --to base64 "$hashAlgo:$hashFromGit")" \
+        --arg hash "$hashFromGit" \
         '.[].ca == {
             method: "git",
             hash: {
                 algorithm: $algo,
-                format: "base64",
+                format: "base16",
                 hash: $hash
             },
         }'