doc: add doc to specify flaky tests on fips

This commit adds documentation lines to BUILDING.md file indicating the
changes that have been made to enable marking flaky tests on fips
compliant system. It is an extension to the prior commit that enables
this behavior.

Fixes: nodejs#14746

BUILDING.md

@@ -135,6 +135,15 @@ $ make test
 At this point you are ready to make code changes and re-run the tests!
 Optionally, continue below.
 
+If you wish to run tests on specific systems, say ``fips``:
+
+```console
+$ make test-on-fips
+```
+
+This particular command will mark the tests specified in the \*.status files as
+flaky.
+
 To run the tests and generate code coverage reports:
 
 ```console