[#781] Fix scheduled steps filtering

Description

Problem: GitLab CI has slightly confusing rules:if behaviour. The step is executed when one of 'if' rules matches unless a rule with 'when: never' is matched. As a result, currently, scheduled docker and testnets pipelines have the same set of steps to execute.

Solution: Use 'if' with 'when: never' and corresponding 'SCHEDULED_*' env variable to filter scheduled steps properly.

See https://docs.gitlab.com/ee/ci/yaml/#rules

Related issue(s)

Fix for #781 (closed)

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