Use SystemRoot env var in explorer path
Use SystemRoot env var in explorer path

Closes atom/atom#4348

jarig/tree-view,matthewbauer/tree-view,jasonhinkle/tree-view,thgaskell/tree-view,atom/tree-view,benjaminRomano/tree-view,tbryant/tree-view,Galactix/tree-view,tomekwi/tree-view,pombredanne/tree-view-1,samu/tree-view,learn-co/learn-ide-tree,ayumi/tree-view,cgrabowski/webgl-studio-tree-view,laituan245/tree-view