std.Build.Step.Run: prefix relative path arguments with './'

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

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