test: set `ReadWritePaths=$BUILD_DIR` when running w/ coverage
    
    otherwise units using `DynamicUser=yes` won't be able to write the
    coverage stats (currently affecting TEST-20-MAINPIDGAMES).
    
    `DynamicUser=yes` implies `ProtectSystem=strict` and
    `ProtectHome=read-only` and can't be overridden hence we need to
    utilize `ReadWritePaths=` to work around that.