ci: set env var NIGHTLY_RUN while checking the apps readme files

sometimes we may want to build & test on only a specific set of targets,
add `NIGHTLY_RUN == "1"` for this use case

tools/ci/check_build_test_rules.py

@@ -518,6 +518,7 @@ if __name__ == '__main__':
                 )
 
         if arg.action == 'check-readmes':
+            os.environ['NIGHTLY_RUN'] = '1'
             check_readme(
                 list(check_dirs),
                 exclude_dirs=_exclude_dirs,