CI: Skip regular jobs from schedules

Thomas Holderrequested to merge
speleo3/inkscape:ci-run_for_both-schedule-never into master

We currently run regular builds 3 times for master, once on "push", once on "source-doc" schedule, and once on "scan-build" schedule.

Example pipelines for latest master, demonstrating the redundant builds:

This MR excludes regular builds from the scheduled pipelines.

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