Ref T12071. Ref T13098. If your "variables_order" does not include "E", you don't get a valid $_ENV.

Detect and automatically repair this. This rarely causes issues in arc, but it can crop up in unusual use cases (see PHI773), and sometimes hg and git care about things.

This doesn't fix the Phabricator case, but we could use a setup warning there more reasonably.

(I can't figure out a better way to repair the value than this one, even though it's gross -- T12071 has some of the things I tried.)

Before patch:

epriestley@orbital ~/dev/arcanist $ php -d variables_order=G -f bin/arc
array(0) {
}
Usage Exception: Choose a workflow!

After patch:

epriestley@orbital ~/dev/arcanist $ php -d variables_order=G -f bin/arc
array(22) {
  ["PHABRICATOR_INSTANCE"]=>
  string(5) "local"
  ["TERM_PROGRAM"]=>
  string(14) "Apple_Terminal"
  ["TERM"]=>
  string(14) "xterm-256color"

...

Usage Exception: Choose a workflow!