[CI] add python environment setup as part of cpp unittest runner script

[leandron]

In #6334, task_cpp_unittest.sh script started also running apps/bundle_deploy's test_dynamic and test_static. If now we try to run task_cpp_unittest.sh as a standalone script, we'll see an error complaining it can't find tvm python module.

The error message is:

(...)
[==========] Running 0 tests from 0 test cases.
[==========] 0 tests from 0 test cases ran. (0 ms total)
[  PASSED  ] 0 tests.
python3 build_model.py -o build --test
Traceback (most recent call last):
  File "build_model.py", line 21, in <module>
    from tvm import relay
ModuleNotFoundError: No module named 'tvm'
Makefile:123: recipe for target 'build/test_graph_c.json' failed
make: *** [build/test_graph_c.json] Error 1

This PR adds the python environment setup script to have TVM's python environment setup available, to make task_cpp_unittest.sh runs as a standalone script.

We don't see this error in the current upstream CI, because we always run the build script before cpp tests, and in this case we'll be in the right directory when this test runs.

cc @areusch @tqchen