Oops, I accidentally wrote a shell in Python

Luigi workflows contain plenty of ad-hoc bash scripts like 'zstdcat | java | pv | zstdmt > output.tmp'.

This is a source of problems, because:

1. a command failing in the script won't fail the script (bash eats errors), so the Python code believes the temporary output was successfully written, commits it, and proceeds: #4773 (closed)
2. most arguments aren't properly escaped
3. a future change to CountPaths will need to build a pipeline dynamically (zstdcat in one case, zstdcat | cat - <(cat other_file.txt | sed) in another, zstdcat | cat <(cat other_file.txt | sed) - in another one), which was be pretty unreadable with string substitutions

For comparison, this is what the change looks like in order modules: https://gitlab.softwareheritage.org/-/snippets/1561 ; and it enables this change: https://gitlab.softwareheritage.org/-/snippets/1562