This is necessary in two cases:
* On POSIX, the exe path (`argv[0]`) must contain a path separator
* Some programs might treat a file named e.g. `-foo` as a flag, which
can be avoided by passing `./-foo`
Rather than detecting these two cases, just always include the prefix;
there's no harm in it.
Also, if the cwd is specified, include it in the manifest. If the user
has set the cwd of a Run step, it is clearly because this affects the
behavior of the executable somehow, so that cwd path should be a part of
the step's manifest.
Resolves: #24216
