Prevent Poetry from trying to read system keyrings

Prevent Poetry from trying to read system keyrings and failing
(specifically reading Windows keyring from an SSH session fails
with "A specified logon session does not exist.")

metapkg/tools/git.py

@@ -4,6 +4,7 @@
     Any,
 )
 
+import os
 import pathlib
 import subprocess
 
@@ -79,7 +80,19 @@ def update_repo(
                 # Origin URL has changed, perform a full clone.
                 clean_checkout = True
 
-    GitBackend.clone(repo_url, revision=ref, clean=clean_checkout)
+    old_keyring_backend = os.environ.get("PYTHON_KEYRING_BACKEND")
+    try:
+        # Prevent Poetry from trying to read system keyrings and failing
+        # (specifically reading Windows keyring from an SSH session fails
+        # with "A specified logon session does not exist.")
+        os.environ["PYTHON_KEYRING_BACKEND"] = "keyring.backends.null.Keyring"
+        GitBackend.clone(repo_url, revision=ref, clean=clean_checkout)
+    finally:
+        if old_keyring_backend is None:
+            os.environ.pop("PYTHON_KEYRING_BACKEND")
+        else:
+            os.environ["PYTHON_KEYRING_BACKEND"] = old_keyring_backend
+
     repo_dir = repodir(repo_url)
     repo = Git(repo_dir)
     args: tuple[str | pathlib.Path, ...]