Add support for running Linux desktop tests on CI

[stuartmorgan-g]

Description

Adds support for building and driving e2e tests for Linux (as a target).

Related Issues

flutter/flutter#55327

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[cyanglaz]

Should we run the tests on stable, too?
Or it is in the future plan, can we add a TODO here?

[stuartmorgan-g]

Like web, stable isn't supported for Linux. I'll add a TODO.

[cyanglaz]

LGTM! CI seems failing

[stuartmorgan-g]

CI seems failing

I'm assuming it's infra failure since it said the agent stopped responding. Trying a re-run, and will escalate to infra channel if it keeps failing.

[stuartmorgan-g]

The failing web tests are using Chrome 81, while the passing master tests are using Chrome 79; my best guess at this point is that the web tests are broken with newer Chrome and my PR is hitting what is essentially submarine bustage because it's rebuilding the Docker image.

[stuartmorgan-g]

@hterkelsen It looks like you added the code for this in #2307; is there a way we can structure this so that we control what version of Chrome we are using for e2e tests instead of random PRs touching the Dockerfile having the side-effect of updating Chrome?

[stuartmorgan-g]

Landing with a modified approach that uses a separate Dockerfile and separate top-level task for the Linux desktop tests, so that progress on Linux isn't gated on the web issue.

At some point when web tests work with the latest Chrome, or the Dockerfile is changed not to rely on a floating Chrome version, they could be re-combined.