fix env null gopath

Author: rwifeng

env.sh

@@ -1,3 +1,8 @@
 #!/usr/local/bin/bash
 
-export GOPATH="${GOPATH}:"$(pwd)
+if [ -z "$GOPATH" ]
+then
+    export GOPATH=$(pwd)
+else
+    export GOPATH="${GOPATH}:"$(pwd)
+fi