Skip backticks not needed by commands

[jayvdb]

If a backtick isnt used by any commands for a testenv, it doesnt need to be evaluated, unless the purpose was to set the envvar for one of the command to read.

There could be a flag which separates envvars needed within the tox.ini vs envvars needed to be passed to the commands.

Potentially filter the list to be passed to the commands could be a separate tox plugin, which this plugin could detect and then it knows which envvars it can eliminate.

[jayvdb]

A way to add support for defining which envvars are needed inside commands is for setenv _FOO= to be considered private to tox , not needed by the commands.

There are very few _ envvars that are used by commands, and they would not be supported by backticks. A non-breaking change later can replace that with a better mechanism if required.