gitlab: convert Cirrus jobs to .base_job_template

gitlab: convert Cirrus jobs to .base_job_template

This folds the Cirrus job rules into the base job
template, introducing two new variables

  - QEMU_JOB_CIRRUS - identifies the job as making
    use of Cirrus CI via cirrus-run

  - QEMU_JOB_OPTIONAL - identifies the job as one
    that is not run by default, primarily due to
    resource constraints. It can be manually invoked
    by users if they wish to validate that scenario.

Signed-off-by: Daniel P. Berrangé <[email protected]>
Message-Id: <20220526110705[email protected]>
Signed-off-by: Alex Bennée <[email protected]>
Reviewed-by: Thomas Huth <[email protected]>
Message-Id: <20220527153603[email protected]>