Set array of envvars at once

[jayvdb]

Rather than using backticks multiple times to set several envvars, there are times when it is more logical and efficient to dynamically emit an array of envvars to be added to setenv.

This is possibly a different tox plugin, at least because it should be done with a different testenv key instead of setenv (indeed, even the backticks could have been implemented using a different key setenv_dynamic).

[jayvdb]

If any item in setenv refers to items created by setenv_dynamic, or inverse, this plugin is the correct place, as the algorithm to determine the correct order of evaluation, and invalidation of old envvars, is similar to backticks.

First chuck of analysis needed is whether/when tox config parser will barf if the setenv list uses a {env:foo} which has not been defined. This will determine when setenv_dynamic must be processed if the setenv list refers to envvar created by setenv_dynamic.