exec-env: unset GOROOT and GOPATH if version is system

Otherwise they -- especially GOROOT -- may end up pointing to a
completely wrong one.

Author: scop

bin/exec-env

@@ -1,6 +1,8 @@
 #!/usr/bin/env bash
 
-if [ "${ASDF_INSTALL_VERSION}" != 'system' ] ; then
+if [ "${ASDF_INSTALL_VERSION}" = 'system' ] ; then
+	unset -v GOROOT GOPATH
+else
 	if [[ "unset" == "${GOROOT:-unset}" ]] ; then
 	    export GOROOT=$ASDF_INSTALL_PATH/go
 	fi