use sh as SHELL for Cygwin/Git-Bash

Author: gucong3000

index.js

@@ -280,7 +280,7 @@ function runCmd_ (cmd, pkg, env, wd, opts, stage, unsafe, uid, gid, cb_) {
 
   if (customShell) {
     sh = customShell
-  } else if (process.platform === 'win32') {
+  } else if (opts.isWindowsShell == null ? process.platform === 'win32' : opts.isWindowsShell) {
     sh = process.env.comspec || 'cmd'
     shFlag = '/d /s /c'
     conf.windowsVerbatimArguments = true