use unique hash engine for each file hasher (#326)

Previously, the factory could cause workers to share a hash engine,
leading to hashes that were wrong or duplicated. This ultimately created
invalid manifests, affecting signature creation and verification.

Signed-off-by: Spencer Schrock <[email protected]>

Author: spencerschrock

src/model_signing/api.py

@@ -176,10 +176,10 @@ def _build_file_hasher_factory(
             The hasher factory that should be used by the active serialization
             method.
         """
-        algorithm = self._build_stream_hasher(hashing_algorithm)
 
         def factory(path: pathlib.Path) -> file.SimpleFileHasher:
-            return file.SimpleFileHasher(path, algorithm, chunk_size=chunk_size)
+            hasher = self._build_stream_hasher(hashing_algorithm)
+            return file.SimpleFileHasher(path, hasher, chunk_size=chunk_size)
 
         return factory