Cirrus: Fix error when GOPATH is unset

testdata/move_to_gopath.bash

@@ -4,10 +4,8 @@ set -euxo pipefail
 shopt -s nullglob globstar
 
 # set GOPATH if empty (travis sets it, but useful for humans)
-if [ -z "$GOPATH" ]; then
+GOPATH="$(go env GOPATH)"
-GOPATH=$(go env GOPATH)
 export GOPATH
-fi
 
 mkdir -p "$GOPATH"/src/github.com/"$CIRRUS_REPO_OWNER"
 cp -av "$PWD" "$GOPATH"/src/github.com/"$CIRRUS_REPO_FULL_NAME"