Fix performanceTest remote fetching

It has been reported that the PerformanceTest fails to fetch the latest master commit when the CI runs from Fork (which can be the case for a VTK-m non-dev submitting a MR in this repo).

This MR fixes this by always fetching from the main repo, represented as a remote named upstream.

This MR also ensures that the Fetch step of the Performance test is ran just once in the ctest invocation, as opposed to the current behavior that runs it before every performanceTest instance. This will also ease issues with the Kitware Gitlab erroring when doing the same git clone repeatedly.