nix-eval-warnings: handle lazy-trees source locations

Nix 2.34 with lazy trees prints stack-trace source locations as
«flakeref»/rel/path.nix:line:col instead of a plain store path, so
extract_source() matched nothing and warnings lost their source,
breaking the package's own test suite in CI (build #395).

Match the guillemet form and return the flake-relative path. Also
annotate re.findall's result to fix a pre-existing mypy no-any-return
error.

Author: Mic92

pkgs/nix-eval-warnings/src/nix_eval_warnings/__init__.py

@@ -89,7 +89,7 @@ def extract_error_message(error_text: str) -> str:
     Looks for the main error message after the stack trace.
     """
     # Look for "error: <message>" pattern - get the last one (most specific)
-    matches = re.findall(r"error:\s*(.+?)(?:\n|$)", error_text)
+    matches: list[str] = re.findall(r"error:\s*(.+?)(?:\n|$)", error_text)
     for match in reversed(matches):
         msg = match.strip()
         # Skip generic messages like "aborting" or stack trace fragments