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

tests/test-rhg.t

@@ -2,8 +2,8 b''
 
 Define an rhg function that will only run if rhg exists
   $ rhg() {
-  > if [ -f "$RUNTESTDIR/../rust/target/debug/rhg" ]; then
-  >   "$RUNTESTDIR/../rust/target/debug/rhg" "$@"
+  > if [ -f "$RUNTESTDIR/../rust/target/release/rhg" ]; then
+  >   "$RUNTESTDIR/../rust/target/release/rhg" "$@"
   > else
   >   echo "skipped: Cannot find rhg. Try to run cargo build in rust/rhg."
   >   exit 80