`cylc play` service: don't unset `$CYLC_ENV_NAME` if no version specified

[MetRonnie]

Follow-up to #525

Unfortunately it broke ~job submission~ cylc play if you were using a ~non-standard Cylc environment name (i.e. not cylc-8.X.Y)~ Cylc environment name that didn't match cylc-8.X.Y exactly, and didn't specify a version in the cylc play-edit form

Check List

• [x] I have read CONTRIBUTING.md and added my name as a Code Contributor.
• [x] Contains logically grouped changes (else tidy your branch by rebase).
• [x] Does not contain off-topic changes (use other PRs for other changes).
• [x] Tests are included
• [x] CHANGES.md entry included if this is a change that can affect users
• [x] No docs needed
• [x] If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[dpmatthews]

a non-standard Cylc environment name (i.e. not cylc-8.X.Y)

The naming convention recommended in the wrapper script is cylc-8.X.Y-Z so this doesn't just affect non-standard names