rhg: Build in release mode on CI
This follows e73b40c790ec which made tests use the release executable.
With e73b40c790ec but not this, tests are skipped on CI
because the executable is missing.
Differential Revision: https://phab.mercurial-scm.org/D9907
.. Common link and substitution definitions.
.. |hg(1)| replace:: **hg**\ (1)
.. _hg(1): hg.1.html
.. |hgrc(5)| replace:: **hgrc**\ (5)
.. _hgrc(5): hgrc.5.html
.. |hgignore(5)| replace:: **hgignore**\ (5)
.. _hgignore(5): hgignore.5.html