Pass -a (--auto-servernum) to xvfb-run so back-to-back calls do not
collide on display :99 while the previous Xvfb is still shutting down.
The newer-R loop added in #216 runs two xvfb-run calls in a row, which
has failed every process-cran-updates-ubuntu-2204-arm64 run since
pipeline 13744 with 'Xvfb failed to start'.