Disable always-authenticate when running under Dependabot (#16773)
Dependabot appears to run a proxy which intercepts all requests and adds
credentials — credentials are _not_ provided via the CLI or environment
variables and there's no way for a user to do so. This means that when
`authenticate = "always"` is used (or when the index URL is on a pyx
domain), uv will fail even though Dependabot may intercept the request
and add credentials.
See
https://github.com/dependabot/dependabot-core/#private-registry-credential-management
