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

• 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).
• Tests are included
• CHANGES.md entry included if this is a change that can affect users
• No docs needed
• 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