Unify main.yml and scheduled.yml, remove 5_8 ref

[rnro]

Motivation:

• main.yml and scheduled.yml are mostly duplicative.
• Scheduled runs failed because of a deprecated reference to a Swift 5.8 pipeline

Modifications:

• Unify main.yml and scheduled.yml
• Remove the reference to the 5.8 pipeline

Result:

Working scheduled runs.