build: fix npm install with --shared

The npm install rules had a hidden dependency on the `node` binary
install rule creating the `$PREFIX/bin` directory.

Because with `./configure --shared` no binary is created, the rule
subsequently failed.  Fix that by creating the directory before
creating the symlinks to the npm and npx scripts.

(Whether it makes sense to install npm without a `node` binary is
a separate question.  This commit is not taking positions. :-))

Regression introduced in commit ed8c89a0 ("build: fix shared installing
target") which, as the commit log indicates, was itself a bug fix for
the `./configure --shared` install.

PR-URL: https://github.com/nodejs/node/pull/16438
Fixes: https://github.com/nodejs/node/issues/16437
Ref: https://github.com/nodejs/node/pull/15148


Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>