`--serial` to execute everything in serial as opposed to parallel

[cadorn]

This is useful for debugging.

Some package install and data fetching tasks run in parallel which may makes bugs harder to track.

If a bug occurs it should be tested with --serial. If it persists the bug does not seem to be a timing issue. If it goes away it may indicate a kink in the implementation.

[cadorn]

Set PINF_SERIAL env var so that --serial carries.