Clarify e2e readme for running on firebase test lab

[CareF]

Description

A few lines update to readme about how to run e2e tests on firebase test lab.

Update for kotlin is still a to-do.

Related Issues

N/A

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[dnfield]

I assume this is to set the desired runtime mode?

If so we should explain that. If not we should explain why we're doing this

[CareF]

I don't think this set the runtime mode. This routine still uploads debug app (which I still need to figure out how to get a release/profile testing app), but flutter build generates some files and directories, e.g. ./gradlew. I'm not sure if there's anything else.

[CareF]

@dnfield PTAL