Write Cloud SQL keyfile_dict credentials with 0600 permissions (#67507)

When the Google connection supplies credentials via ``keyfile_dict``,
``CloudSqlProxyRunner._get_credential_parameters`` wrote the credentials
file with ``open(path, "w")``. That inherits the process umask
(typically ``0o644`` on most distributions), leaving the service-account
private key world-readable on shared worker hosts — including any
other process on the same machine that can read the worker's temp
directory.

Use ``os.open(..., O_WRONLY | O_CREAT | O_TRUNC, 0o600)`` followed by
``os.fdopen`` so the file is created with restrictive permissions
atomically. Matches the explicit-mode handling already used for the
SSL temp files in the same module.