Do not keep UEFS mounted engines mounted across build runs
This continues to cause intermittent issues for some reason. The underlying cause will be some bug in the storage virtualization/VHD layer, but right now we just need to get builds stable. Thus we should not keep mounts across build runs and instead have a fresh engine every time.
The downside to this is that the clang-tidy plugin that we use for validating other plugins won't be installed properly on each build step, which is not really ideal. We should copy the clang-tidy installation code into UET with a special flag or environment variable (with the clang-tidy binaries still provided under their original license for now) so that we can have this working for our own build system.