chore(ci): add manual trigger

[BobrImperator]

• Adds a workflow_dispatch trigger so the CI can be manually triggered
• Adds a schedule trigger to run at the beginning of each month.

Needed for one-off artifact creation for use in https://github.com/mainmatter/rust-exercises.com
We might want to consider adding a schedule too because artifacts expire after 90 days.

https://github.com/mainmatter/rust-advanced-testing-workshop could use the same config.

[LukeMathWalker]

A schedule would be ideal, with a reasonably long step.

[BobrImperator]

Added a schedule to run at the beginning of each month.
Kept the workflow dispatch too, could be useful later if something doesn't work still.