The previous solution forced us to switch to an entirely new OpenVDB branch, which may not be desireable.
But NanoVDB is header-only and backwards compatible with older OpenVDB versions to a certain degree, so this patch makes it a standalone dependency that is independent of the OpenVDB one.

It works by downloading the "feature/nanovdb" branch of OpenVDB, but using the NanoVDB CMake in the "nanovdb" subdirectory, instead of the root one and therefore ignoring everything else.
Also enabled NanoVDB by default for make deps builds now.