Restrict --set & --set-file options in cylc vr

[wxtim]

This PR documents the bug as well as fixing it. There is no related issue. Follow-up to #5229

Synopsis

--set and --set-file were included by mistake in cylc vr because I copied the general form of VR from cylc vip where it makes sense. Reinstall doesn't use these CLI variables so they won't change the workflow. This is potentially misleading to users.

Code to try

#!jinja2
# Cylc Workflow
[scheduler]
    allow implicit tasks = ture
[scheduling]
    [[graph]]
        {{CYCLE}}= foo => bar => baz

then,

On the command line

cylc vip --set 'CYCLE="PT1H"'
...

# Does not change the running workflow!
cylc vr --set 'CYCLE="PT2H"' 

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg and conda-environment.yml.
• No tests, change is specification rather than tests.
• No CHANGES.md entry - this effectively fixes a recently added PR before relase.
• NO Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.

[oliver-sanders]

a