Run integration tests on `ubuntu-latest` instead of `macos-latest`

The ubuntu runner typically performs better than the macos runner, so switching to ubuntu could improve our test runtimes.

An attempt at this is in this closed PR here: https://github.com/flutter/devtools/pull/6569

The integration tests with a flutter web device failed with both the `-d chrome` and `-d web-server`.

https://github.com/flutter/devtools/actions/runs/6616578190/job/17971202845?pr=6569

This will need to be investigated in order to switch the tests over to ubuntu. Another option is to run just the flutter web integration tests on macos and run the rest on ubuntu but this would require some refactoring of how we define our integration tests workflow.

Review the integration-test workflow and the closed PR #6569, then inspect the failed GitHub Actions job for the Flutter web devices `-d chrome` and `-d web-server`. Determine why those tests fail on `ubuntu-latest`; done means the integration tests run successfully on Ubuntu, or the workflow cleanly separates web tests on macOS from the remaining tests on Ubuntu.

Scritto dal modello di indicizzazione a partire dal testo della issue.