rhg: use a release-mode executable in tests This allows the rhg build for test-rhg.t to share compiled dependencies such as hg-core with the hg-cpython build for other tests. For context, my wrapper script for the typical edit-compile-test cycle now looks like this: (cd rust && cargo +nightly-2020-10-04 fmt) (cd rust && cargo build --release -p rhg) make --silent local PURE=--rust python test/run-tests.py --local "$@" Differential Revision: https://phab.mercurial-scm.org/D9728