All non-root tests fail on GPU machine

A recent addition to ensure that NvidiaVolume::create() ran as root broke all non-root tests on GPU machines. The reason is that we unconditionally create this volume so long as we detect nvml.isAvailable() which will fail now that we are only allowed to create this volume if we have root permissions.

We should fix this by adding the proper conditions to determine when / if we should create this volume based on some combination of --containerizer and --isolation flags.