Use SRI hash (strings) as the official JSON format for Hash after all

The fact that we were introducing a conversion from the output of `nix
path-info` into the input of `builtins.fetchTree` was the deciding
factor. We want scripting outputs into inputs like that to be easy.

Since JSON strings and objects are trivially distinguishable, we still
have the option of introducing the JSON format as an alternative input
scheme in the future, should we want to. (The output format would still
be SRI in that case, presumably.)

tests/functional/fixed.sh

@@ -17,11 +17,7 @@ nix-build fixed.nix -A bad --no-out-link && fail "should fail"
 nix path-info --json --json-format 2 "$path" | jq -e \
     '.info.[].ca == {
         method: "flat",
-        hash: {
-            algorithm: "md5",
-            format: "base16",
-            hash: "8ddd8be4b179a529afa5f2ffae4b9858"
-        },
+        hash: "md5-jd2L5LF5pSmvpfL/rkuYWA=="
     }'
 
 echo 'testing good...'