Revert moving executor init before VU init
Revert moving executor init before VU init

This was done in e693715b to ensure exec validation would fail before
VUs are initialized, but now that this validation is done much earlier,
this is no longer needed.

Resolves https://github.com/loadimpact/k6/pull/1428#discussion_r422157781

loadimpact/k6,loadimpact/k6