Fix test workflow logic

[wxtim]

Issue

Automated workflow syntax warning:

Workflow syntax warning: .github/workflows/test_cylc.yml#L25
.github/workflows/test_cylc.yml (Line: 25, Col: 13): Conditional expression contains literal text outside replacement tokens. This will cause the expression to always evaluate to truthy. Did you mean to put the entire expression inside ${{ }}?

Upon editing live in GH the context sensitive help pointed to the if: directive not requiring the ${{ }} around `var = "test" because that's pretty much what an if statement would expect anyway.

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.