Fix possible silent failure to connect to workflow

[MetRonnie]

If you played a workflow with the same workflow ID as a previously existing one, but a new UUID, the UI Server was not connecting to it.

Due to cylc/cylc-flow#5615 this was affecting restarting any workflow.

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).
• No dependency changes
• Tests are included (or explain why tests are not needed).
• 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.

[MetRonnie]

Not sure about testing this one

[codecov-commenter]

Codecov Report

Patch coverage: 68.75% and project coverage change: +0.08 🎉

Comparison is base (929714a) 76.96% compared to head (ec3f47e) 77.05%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #463      +/-   ##
==========================================
+ Coverage   76.96%   77.05%   +0.08%     
==========================================
  Files          12       12              
  Lines        1385     1412      +27     
  Branches      236      239       +3     
==========================================
+ Hits         1066     1088      +22     
- Misses        276      277       +1     
- Partials       43       47       +4     

Impacted Files	Coverage Δ
cylc/uiserver/workflows_mgr.py	83.23% <68.75%> (-0.89%)	⬇️

... and 6 files with indirect coverage changes

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