Remove the build folder for unit test to ensure correctness

[YuanTingHsieh]

Fix the occasionally issue in TestTaskScriptRunner

Description

Sometimes the unit test failed because there is a "build" dir, this will lead to test failure:

AssertionError: Lists differ: ['/ho[28 chars]VFlare-premerge/build/lib/nvflare/cli.py', '--batch_size', '4'] != ['/ho[28 chars]VFlare-premerge/nvflare/cli.py', '--batch_size', '4']

That build dir is generated by pip install and can be removed during the unit test phase to ensure correctness

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Quick tests passed locally by running ./runtest.sh.
• In-line docstrings updated.
• Documentation updated.

[YuanTingHsieh]

/build