BDE tools does not install a build into the configured installation prefix

I'm trying to use the BDE build system similarly to the interfaces of the build systems of most other open source software I've built. Which is the standard dance:
```
./configure --prefix=/home/my/bde
make
make install
```

I've translated that into:
```
eval $(bde_build_env)
bbs_build configure --prefix /home/my/bde
bbs_build build
bbs_build install
```

However, this results in files installed into `/home/my/bde/opt/bb` (notice the `opt/bb`). How do I prevent the BDE build system from decorating my prefix with `opt/bb`?

In general, this tool should have usage that aligns well with open-source expectations. I'm aware of the relocatable installation requirement (i.e. support for distribution "refroots") but this behavior is confusing. The prefix I specify should be the prefix into which the artifacts are installed.

**Environment:**
- Darwin G7WH7WFVKV 21.6.0 Darwin Kernel Version 21.6.0

Start by tracing the bbs_build configure, build, and install entry points after bde_build_env sets the environment. Reproduce the commands from the issue with the configured prefix and identify where opt/bb is added. Done means installation places artifacts directly under the requested prefix while preserving relocatable-installation behavior.

Written by the indexing model from the issue text.