cylc.gcapture: don't split command

[matthewrmshin]

Address issue similar to the #3183 PR, but on commands such as cylc trigger --edit via the GUI.

Requirements 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).
• GTK logic currently has no automated test.
• Includes an appropriate entry in the release change log CHANGES.md.

Reported by @steoxley.

[hjoliver]

Tested as working.

[hjoliver]

@MartinRyan - a) I suppose you might want to check the security implications of us having to revert your "split command" changes? and b) for future reference, any change not covered by automated tests - such as any change to our old GUI! - must be manually tested (and that's directed at myself and other reviewers as much as at you ... since we failed to pick this up in review!)

[kinow]

Note taken about manual testing. I believe that applies for the future Web GUI too. Whenever we cannot have automated tests, we must remember to do some manual testing 👍 will try to remember that too. Thanks!