builder: fix escaping for ENV variables.

/cc @tiborvass 

closes #8636 
OH, I thought I fixed that. One minute, thanks.

-Erik

> On Oct 17, 2014, at 2:32 PM, Alexandr Morozov <notifications@github.com> wrote:
> 
> In builder/support.go:
> 
> >  	// this intentionally punts on more exotic interpolations like ${SOME_VAR%suffix} and lets the shell handle those directly
> >  )
> >  
> >  // handle environment replacement. Used in dispatcher.
> >  func (b *Builder) replaceEnv(str string) string {
> >  	for _, match := range tokenEnvInterpolation.FindAllString(str, -1) {
> > +		if strings.Index(match, "\\$") != -1 {
> > +			idx := strings.Index(match, "\\$")
> Why you need two same calls? Maybe you should find idx first?
> 
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