Cylc Review: support alternate home dirs.

[hjoliver]

This is a minimal change to support viewing symlinked run dirs directly instead of via $HOME.

At ESNZ our HPC home directories now have to be private, but we have shared project dirs at the end of our cylc install symlinks. This change allows a Cylc Review service running as a project member to see the run dirs of all project members, via the symlinked dirs instead of $HOME.

From cylc review --help on this branch:

If $CYLC_REVIEW_HOME is defined, look there for run directories instead of
of user home directories. This can be used to view symlinked run directories
directly when home directories are private. For example with symlinking as:
    /home/USER/cylc-run/WORKFLOW -> /project/PROJECT/USER/cylc-run/WORKFLOW
set the environment variable as:
    CYLC_REVIEW_HOME=/project/PROJECT

There were several places in the code where home dirs were assumed. To avoid any chance of breaking existing use cases, I just banged handling of $CYLC_REVIEW_HOME in first, in those locations: if the variable exists, do it the new way; else do it the old way.

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 if present).
• Tests are included (or explain why tests are not needed).
• Changelog entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.