stem: use Cylc host selection interface

[oliver-sanders]

• The ROSE_ORIG_HOST variable is set using the Cylc interface.
• The HOST_SOURCE... rose-stem variables are set using the Rose interface.
• The two interfaces differ, Cylc picks long hostnames, Rose picks short ones.
• The inconsistency may cause issues with simplistic "is this localhost" checks.
• The shortnames are causing issues at one site.
• Switch to using the Cylc interface for all scenarios.

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.