slugbuilder: Set CF_STACK environment variable
slugbuilder: Set CF_STACK environment variable

This is required for the staticfile buildpack.

Signed-off-by: Jonathan Rudenberg <3692bfa45759a67d83aedf0045f6cb635a966abf@titanous.com>

benbjohnson/flynn,pkdevbox/flynn,philiplb/flynn,Brandywine2161/flynn,pkdevbox/flynn,benbjohnson/flynn,schatt/flynn,whouses/flynn,GrimDerp/flynn,clifff/flynn,kgrz/flynn,pkdevbox/flynn,josephglanville/flynn,clifff/flynn,jzila/flynn,justintung/flynn,GrimDerp/flynn,pkdevbox/flynn,supermario/flynn,justintung/flynn,josephglanville/flynn,schatt/flynn,schatt/flynn,rikur/flynn,jzila/flynn,ozum/flynn,rikur/flynn,TribeMedia/flynn,philiplb/flynn,schatt/flynn,lmars/flynn,supermario/flynn,Brandywine2161/flynn,whouses/flynn,josephglanville/flynn,whouses/flynn,flynn/flynn,benbjohnson/flynn,benbjohnson/flynn,josephglanville/flynn,kgrz/flynn,flynn/flynn,kgrz/flynn,lmars/flynn,pkdevbox/flynn,shads196770/flynn,rikur/flynn,GrimDerp/flynn,kgrz/flynn,ozum/flynn,Brandywine2161/flynn,clifff/flynn,benbjohnson/flynn,lmars/flynn,justintung/flynn,clifff/flynn,Brandywine2161/flynn,GrimDerp/flynn,Brandywine2161/flynn,clifff/flynn,philiplb/flynn,jzila/flynn,shads196770/flynn,clifff/flynn,schatt/flynn,schatt/flynn,jzila/flynn,shads196770/flynn,philiplb/flynn,lmars/flynn,josephglanville/flynn,justintung/flynn,supermario/flynn,TribeMedia/flynn,ozum/flynn,supermario/flynn,supermario/flynn,GrimDerp/flynn,flynn/flynn,pkdevbox/flynn,TribeMedia/flynn,rikur/flynn,kgrz/flynn,lmars/flynn,ozum/flynn,Brandywine2161/flynn,benbjohnson/flynn,whouses/flynn,justintung/flynn,jzila/flynn,shads196770/flynn,whouses/flynn,josephglanville/flynn,rikur/flynn,flynn/flynn,jzila/flynn,shads196770/flynn,justintung/flynn,ozum/flynn,whouses/flynn,TribeMedia/flynn,flynn/flynn,GrimDerp/flynn,supermario/flynn,rikur/flynn,TribeMedia/flynn,ozum/flynn,philiplb/flynn,TribeMedia/flynn,shads196770/flynn,kgrz/flynn