tests: fix fetchers-substitute test for new narHash JSON format

The test was failing because nix path-info --json now returns narHash as
a structured dictionary {"algorithm": "sha256", "format": "base64",
"hash": "..."} instead of an SRI string "sha256-...".

This change was introduced in commit 5e7ee80. The functional test
path-info.sh was updated at that time, but this NixOS test was missed.

The fix converts the dictionary format to SRI format inline:
  tarball_hash_sri = f"{narHash_obj['algorithm']}-{narHash_obj['hash']}"

Author: lovesegfault

tests/nixos/fetchers-substitute.nix

@@ -120,7 +120,9 @@
       path_info_json = substituter.succeed(f"nix path-info --json {tarball_store_path}").strip()
       path_info_dict = json.loads(path_info_json)
       # nix path-info returns a dict with store paths as keys
-      tarball_hash_sri = path_info_dict[tarball_store_path]["narHash"]
+      narHash_obj = path_info_dict[tarball_store_path]["narHash"]
+      # Convert from structured format {"algorithm": "sha256", "format": "base64", "hash": "..."} to SRI string
+      tarball_hash_sri = f"{narHash_obj['algorithm']}-{narHash_obj['hash']}"
       print(f"Tarball NAR hash (SRI): {tarball_hash_sri}")
 
       # Also get the old format hash for fetchTarball (which uses sha256 parameter)