make sure GOROOT is always set (fixes #13), because the invoked gcov2lcov uses golang API functions that rely on GOROOT (i.e. build.Import, which calls https://cs.opensource.google/go/go/+/refs/tags/go1.18.2:src/go/build/build.go;drc=0a1a092c4b56a1d4033372fbd07924dad8cbb50b;l=544)
Up to version v3 of the github setup-go action, the GOROOT variable was automatically set. But that changed here, and that's why we nowmak sure GOROOT is always set.