[std] Fix std.addRunnable() possibly clobbering files in brioche-run.d

[kylewlacy]

When std.addRunnable() is called, it assumes nothing else is under brioche-run.d, and will add recipes to this folder starting with the name recipe-0. This can cause problems if std.addRunnable() is called more than once. We need a scheme to add files in a way where calling std.addRunnable() more than once uses separate paths for each runnable

[kylewlacy]

One possible scheme: we could make it so std.addRunnable(recipe, "bin/path", { ... }) puts runnable files under brioche-run.d/bin/path/...