Eio.Process: match Unix search behaviour
Don't search `$PATH` if the name contains a `/`.
https://pubs.opengroup.org/onlinepubs/9799919799/ says:
> If the file argument contains a <slash> character, the file argument
> shall be used as the pathname for this file. Otherwise, the path
> prefix for this file is obtained by a search of the directories passed
> as the environment variable PATH