It's better to avoid references to this global variable, but, in the
cases where it's needed, such as in std.debug.print and collecting stack
traces, better to share the same instance.
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
Andrew Kelley