ForceRun runs 'exe [args...]' in current working directory, and d.Chk()s on failure.

ForceRunInDir runs 'exe [args...]' in the given directory, and d.Chk()s on failure.

RunInDir runs 'exe [args...]' in the given directory, returning any failure.

Serial serially runs all instances of filename found under dir, mapping stdout and stderr to each subprocess in the obvious way.

Env is a map of env vars, mapping key string to value string.