Allow use of ^ and $ in actions like trigger

[ColemanTom]

Problem

Discussed in https://cylc.discourse.group/t/rerun-an-r1-task-many-cycles-later/1084/4 - to trigger a task at the first or last cycle point requires you to know what those cycles are.

Proposed Solution

Allow use of ^ and $ for actions like cylc trigger.

[hjoliver]

(This one is actually quite important - having to figure out the initial cycle point is potentially quite painful for operators)

[ColemanTom]

(This one is actually quite important - having to figure out the initial cycle point is potentially quite painful for operators)

Whilst I raised this, other than through the cylc config command, the initial cycle (not sure about final cycle) seems to be printed at the top of every rotation of the scheduler log, so it should not be hard for anyone to find it out via manual or automated means.