build: skip long-running Actions for README-only modifications

If the only file modified is README.md do not run test-linux or
build-windows tasks. This will help streamline onboarding sessions but
may help some other cases too.

PR-URL: https://github.com/nodejs/node/pull/40571


Reviewed-By: Voltrex <mohammadkeyvanzade94@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>