Use 'nix flake prefetch' to get the store path of a flake

Future-proofing for when 'nix flake metadata' won't return a store
path anymore.

Author: edolstra

tests/functional/flakes/flake-in-submodule.sh

@@ -74,7 +74,7 @@ EOF
 git -C "$rootRepo" add flake.nix
 git -C "$rootRepo" commit -m "Add flake.nix"
 
-storePath=$(nix flake metadata --json "$rootRepo?submodules=1" | jq -r .path)
+storePath=$(nix flake prefetch --json "$rootRepo?submodules=1" | jq -r .storePath)
 [[ -e "$storePath/submodule" ]]
 
 # The root repo may use the submodule repo as an input