Disallow nested installation dirs

[MetRonnie]

Somewhat related to #4394

We should probably not allow installation into a subdir of an installation dir (a dir that contains _cylc-install). E.g. if you have

~/cylc-run
`-- foo
    |-- _cylc-install
    `-- run1

then you should not be able to do cylc install --flow-name foo/bar because this will result in nested installation dirs:

~/cylc-run
`-- foo
    |-- _cylc-install
    |-- run1
    `-- bar
        |-- _cylc-install
        `-- run1

(here we have the installation dir foo/bar nested inside the installation dir foo.

Note that nested run dirs are already disallowed. We might be able to modify this function for installation dirs:

cylc-flow/cylc/flow/workflow_files.py

Line 1230 in 4735043

def check_nested_run_dirs(run_dir: Union[Path, str], flow_name: str) -> None:

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).

[oliver-sanders]

Note that the run dirs aren't themselves nested so this isn't a problem for workflow identification. The issue is the mapping of run and src dirs provided by the _cylc-install/source symlink.

[hjoliver]

Decision: don't allow nesting. Checking has to go up and down.