pin bop_toolkit version

### Describe your feature request

recent updates broke installation within blenderproc:
https://github.com/thodan/bop_toolkit/issues/194

### Describe a possible solution

instead of blindly pulling master, rather pin a working commit here:
https://github.com/DLR-RM/BlenderProc/blob/1dfe3b1c799233b57feb5753ad9cfb1cd3ac86a5/blenderproc/python/writer/BopWriterUtility.py#L140

e.g.
```
git+https://github.com/thodan/bop_toolkit@fcb9834ccc9a46dcdd3306f27eb003f6ba21b60e
```

Start at blenderproc/python/writer/BopWriterUtility.py around line 140 and inspect how the bop_toolkit dependency is fetched. Pin it to the working commit named in the issue, then verify that BlenderProc installation no longer pulls incompatible updates.

Written by the indexing model from the issue text.