Fix web-dashboard output conflict

[inancgumus]

What?

Fix web-dashboard output conflict.

Why?

Running:

K6_WEB_DASHBOARD=true k6 run --out web-dashboard script.js

Results:

...listen tcp :port: bind: address already in use

Also see #4707.

Checklist

• [x] I have performed a self-review of my code.
• [x] I have commented on my code, particularly in hard-to-understand areas.
• [x] I have added tests for my changes.
• [x] I have run linter and tests locally (make check) and all pass.

Checklist: Documentation (only for k6 maintainers and if relevant)

Please do not merge this PR until the following items are filled out.

• [x] I have added the correct milestone and labels to the PR.
• [ ] I have updated the release notes: link

Related PR(s)/Issue(s)

Closes #4707