Skip installing xvfb and chromium in CI

[janbrasna]

Description

One of the less reliable things in GHA here is apt-install of things. Last time it took 120min+ to install chromium at mere 25kBps, but even on a good day the usual apt-get update && apt-get install chromium-browser xvfb spends time pulling manifests, restarting daemons etc., setting up snapd, and then installing the apps… without any caching always relying on canonical infra providing these… usually taking at least 2mins, but often spending several times longer on a step that is probably not even necessary:

1. Both xvfb and chromium are however already preinstalled in the runner, and don't need installing or updating. (Their versions are kept up-to-date via weekly runner image rollouts, but otherwise consistent between the runs helping reproducibility.)
2. Looking into what jasmine runs, I even doubt the installed chromium is ever used afterwards — one set runs system firefox via geckodriver, the other runs system chrome via matching chromedriver version, all preinstalled on the runner.

Quick PoC shows the action runs fine without any additional installs.

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Success Criteria

• Reproducible versions are preferred instead of always pulling the latest over and over
• Tooling uses whatever binaries are available on the runner, without overriding the installs
• FE tests finish twice as fast

[janbrasna]

Next:

• follow subsequent stage+prod pushes there's no rules relying on the naming
• watch a few runs from different triggers to verify caching efficiency/reuse
• port to fxc when happy

[stevejalim]

follow subsequent stage+prod pushes there's no rules relying on the naming

I think we're pretty safe here -- it's only the integration-test and static-asset-check YAML files that are used as part of webhook URLs - the unit tests / "pull request tests" are only triggered by push events

[janbrasna]

I remember GH's "branch protection rules" having actually the run name referenced (which is kept here, no matter the rename), so even if there is something that insists on main green before pushing/tagging elsewhere, chances are it will still continue to work. What chronically breaks on renames are the "CI status" badges that are not used here really;)