EVG-17874: Update Genny with --no-activate flag.

[johndaniels]

This allows us to create task generation tasks that don't immediately execute all the generated variants.

It's a bit awkward to pass through a flag that is phrased in the negative (as opposed to using an "activate" flag and variable), but in this case we need to because an explicit activate flag would have different behavior than the default of not specifying the "activate" parameter. If the "activate" parameter is present on a task, then that task is always activated, but if it is not present then it defaults to if the version is activated or through batchtime.

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[thessem]

It is going to take me a little bit of time to be able to review this one. There are a few things going on (unplumbing of genny_repo_root which doesn't seem to have been used anyway, modification of exec_timeout_secs in a test without any non-test code seemingly breaking? then again, CI is failing right now....) that are probably obvious with enough context