reload: allow template variables to change on reload

[oliver-sanders]

We allow template variables to change when we restart a workflow, but not when we reload one.

The cylc vr command will either:

• Restart the workflow if it is stopped.
• Reload the workflow if it is running.

Since cylc restart accepts template variables and cylc reload does not, cylc vr will print a warning if template variables are set, but the workflow is running (since it cannot enact this change).

This restriction is artificial (no fundamental reason why we can't allow template variables to be changed on reload), and appears to have caused a bit of confusion: #5965.

For functionality / consistency, we should support setting template variables with the cylc reload command the same as for cylc play.

[oliver-sanders]

Note, from #5965

Remember to remove the warning from cylc vr when implementing this.