Previously, the `ci.py` script would execute all C++ unit tests in the
`"build"` directory, regardless of the docker image being used.  This
change allows a caller to specify the build directory to be used by
`task_cpp_unittest.sh`, either by the command line or by using the
same `TVM_BUILD_PATH environment variable as used by the top-level
Makefile, and passes this argument from `ci.py`.  To preserve the
existing behavior for the pre-commit CI, if no argument is passed and
if the `TVM_BUILD_PATH` is undefined, `task_cpp_unittest.sh` defaults
to the `"build"` directory.

Python unit tests executed through `ci.py` used the `TVM_LIBRARY_PATH`
environment variable, and were not similarly affected.

Co-authored-by: driazati <9407960+driazati@users.noreply.github.com>

Otherwise, `set -u` rightly errors out for it being undefined.