Fallback format via environment variable

Currently format fallback only works for a hardcoded list of processes. Would it be possible to add an environment variable that the parent process can set to set the fallback format?

Start by locating the existing fallback-format handling and the hardcoded process list in the C sources. Determine how a parent-provided environment variable should supply the fallback format, then add coverage for the requested behavior if the repository has a relevant test location.

Written by the indexing model from the issue text.