ci: Add support for ONLY_JOBS variable to trigger any validation pipeline

By setting the ONLY_JOBS variable to the name of the job (or multiple
jobs), the resulting
pipeline will include a validation job for that pipeline.

For example - if you set ONLY_JOBS="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.

Fixes #25332