Make sure to pass the directory you installed re2 on build

What does this Merge Request do and why?

Makes sure to pass the directory you installed re2 on build to avoid errors on bundle install (gem install re2) on Apple M1.

With re2 1.2.0 in gitlab-org/gitlab, we need this workaround while not need with re 1.3.0+. cf. https://github.com/mudge/re2/issues/50

Why

re2 is installed under /opt/homebrew/opt/pcre2 via brew install re2 on M1 Mac instead of /usr/local as of 2021-05-20. Upgrading re2 Gem to 1.3.0+ (latest 1.4.0) resolves this workaround in the future.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.

Closes #1225 (closed)