Disallow runX as a workflow name

[MetRonnie]

As described by Bruno on Element, cylc install --flow-name=foo/run4 results in a run directory ~/cylc-run/foo/run4/run1 which is confusing, so should probably be disallowed. This should mean disallowing names matching the regex run\d+ (I think runN is already disallowed, but need to check that) and runN.

Pull requests welcome!
This is an Open Source project - please consider contributing a bug fix
yourself (please read CONTRIBUTING.md before starting any work though).

[kinow]

(I think runN is already disallowed, but need to check that).

You are allowed to install a/b/c/runN. It creates the folders, but somewhere down the line Cylc Flow understands that is invalid and doesn't show it on cylc scan.

(venv) kinow@ranma:~/cylc-src/three$ cylc install --flow-name=a/b/c/runN
INSTALLED a/b/c/runN/run1 from /home/kinow/cylc-src/three
(venv) kinow@ranma:~/cylc-src/three$ cylc scan --state=all
five
747
three/run8
a/run1
a/b/run1
one/two/three/run2
one/two/three/run4 ranma:43010
one/two/three/run3 ranma:43023
one/two/three/run1 ranma:43085