tools: ignore build folder when checking links

We checkout build as a subdirectory as part of CI and
if you run `make test` instead of `make test-ci` you get
loads of errors about markdown link breaks. Ignore this
directory as we don't need to examine another repo

PR-URL: https://github.com/nodejs/node/pull/35315


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>