build: do not cd on vcbuild help

`vcbuild help` just outputs help info and exits.

If a user calls this command not from a project root,
the directory change can be unexpected and unwanted.

PR-URL: https://github.com/nodejs/node/pull/19291


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>