test(mirage): correct server authority string in preview builds (backport #1283)

[mergify[bot]]

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Fixes: #1271

How to manually test:

1. Check out PR
2. yarn start:dev:preview should bring up the preview/demo UI. There should be a Fake Target, with a Demo template. Topology view should work, dashboard should work, recording start/stop/archive/delete should work, expanding recording for automated analysis should work. GraphQL-powered things will not work, snapshot recording creation does not work.
3. yarn run build:preview:notests && cd dist && python3 -m http.server, then open http://localhost:8000 in a browser. The preview should work just the same as in the step above.

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   This is an automatic backport of pull request #1283 done by Mergify.

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