feat: display keyring stderr (#4343)
## Summary
Closes https://github.com/astral-sh/uv/issues/4162
Changes keyring subprocess to allow display of stderr.
This aligns with pip's behavior since pip 23.1.
## Test Plan
* Tested using gnome-keyring-backend on a self-hosted private registry
as well as the keyring script described in #4162 to confirm both
existing functionality and the new stderr display.
* Existing tests using `scripts/packages/keyring_test_plugin` are now
showing its stderr output as well.
