ci: Allow triggering individual pipelines

There are a variety of labels which can be used to trigger different validation pipelines.. but sometimes you want to trigger a specific one which you can't control with a label.

Therefore we should introduce an environment variable which you can use when triggering a pipeline in order to make sure a specific job is included. For example - if you set ONLY_JOB="x86_64-linux-ubuntu22_04-validate" then a ubuntu22_04 job will be included in the validation pipeline. This is useful for testing specific jobs.