ci: require PYCLOUDLIB_TOML and SSH key secrets, write config to runner temp

- Declare PYCLOUDLIB_TOML, SSH_PRIVATE_KEY, and SSH_PUBLIC_KEY as
  required secrets on the workflow_call trigger.
- Add an early assertion step that exits immediately if
  PYCLOUDLIB_TOML is empty.
- Replace the ssh-keygen throwaway key with a new 'Setup SSH' step
  that writes the injected public/private key pair to ~/.ssh, for use
  in authentication to instances under test
- base64-decode PYCLOUDLIB_TOML secret and write it to
  $RUNNER_TEMP/pycloudlib.toml to keep secrets off the persistent filesystem.
  Add cleanup to remove the file unconditionally on exit.