Update scripts to pass Shellcheck lints (#11166)

.ci/scripts/test_export_data_command.sh

.ci/scripts/test_synapse_port_db.sh

changelog.d/11166.misc

contrib/purge_api/purge_history.sh

debian/build_virtualenv

debian/changelog

debian/matrix-synapse-py3.config

debian/matrix-synapse-py3.postinst

debian/test/provision.sh

demo/clean.sh

demo/start.sh

demo/stop.sh

docker/build_debian.sh

scripts-dev/check-newsfragment

scripts-dev/check_line_terminators.sh

scripts-dev/complement.sh

scripts-dev/config-lint.sh

scripts-dev/docker_update_debian_changelog.sh

scripts-dev/generate_sample_config

scripts-dev/next_github_number.sh