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

contrib/heptapod-ci.yml

@@ -32,7 +32,7 b' variables:'
       - hg -R /tmp/mercurial-ci/ update `hg log --rev '.' --template '{node}'`
       - ls -1 tests/test-check-*.* > /tmp/check-tests.txt
       - cd /tmp/mercurial-ci/rust/rhg
-      - cargo build
+      - cargo build --release
       - cd /tmp/mercurial-ci/