build: set GO15VENDOREXPERIMENT in env.sh

This should fix the 'make' build with Go 1.5.

build/env.sh

@@ -20,7 +20,8 @@ fi
 
 # Set up the environment to use the workspace.
 GOPATH="$workspace"
-export GOPATH
+GO15VENDOREXPERIMENT=1
+export GOPATH GO15VENDOREXPERIMENT
 
 # Run the command inside the workspace.
 cd "$ethdir/go-ethereum"