doc: remove "multiple variants" from BUILDING.md

The statement that tests for tier 1 platforms are run on multiple
variants is not true. We usually only run on one variant of macOS.
Remove "multiple", which simplifies and clarifies the statements anyway.

PR-URL: https://github.com/nodejs/node/pull/30366


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>