[BUILD] Allow building openvdb in a super project with TBB on its side

### Environment
**Operating System:** all
**Version / Commit SHA:** 36898a
**CMake Version:** 3.20
**Compiler:** all

### Describe the problem
If I put openvdb in a larger project with tbb on its side and use `add_subdirectory` for both of them and it still tries to find_package(TBB) even though the target is already there. The solution is quite simple and consists of adding a

` if (NOT TARGET TBB::tbb)` on line 123 of `openvdb/openvdb/CMakeLists.txt` so that the code becomes:

```
if (NOT TARGET TBB::tbb)
find_package(TBB ${MINIMUM_TBB_VERSION} REQUIRED COMPONENTS tbb)
if(OPENVDB_FUTURE_DEPRECATION AND FUTURE_MINIMUM_TBB_VERSION)
if(${Tbb_VERSION} VERSION_LESS FUTURE_MINIMUM_TBB_VERSION)
message(DEPRECATION "Support for TBB versions < ${FUTURE_MINIMUM_TBB_VERSION} "
"is deprecated and will be removed.")
endif()
endif()
endif()
```

### To Reproduce
Steps to reproduce the behavior:
1. Checkout repo '...'
2. Run CMake '....'
3. Build with '....'
4. See error

### Additional context
(Add any other context about the problem here.)

Start in openvdb/openvdb/CMakeLists.txt around line 123 and inspect how the TBB dependency is discovered when OpenVDB and TBB are added as sibling subdirectories. Reproduce the configuration with both projects present, then verify that an existing TBB::tbb target avoids the unnecessary package lookup and that the project configures successfully.

Written by the indexing model from the issue text.