doc: simplify and clarify FIPS text in BUILDING.md

PR-URL: https://github.com/nodejs/node/pull/17538
Fixes: https://github.com/nodejs/node/issues/17536


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>