mypy: Exclude build directory

Otherwise we get errors of the form

Finished executing pythonRemoveTestsDir
running install tests
ghc_triage_service/__init__.py: error:(B Duplicate module named 'ghc_triage_service' (also at './build/lib/ghc_triage_service/__init__.py')(B
ghc_triage_service/__init__.py: note:(B Are you missing an __init__.py? Alternatively, consider using --exclude to avoid checking one of them.
Found 1 error in 1 file (errors prevented further checking)(B