ci(dgb): configure smoke with -DCOIN_DGB=ON so test subtree builds

The Phase-B smoke gate configured with -DBUILD_TESTING=ON only; the dgb
test subtree is gated on if(COIN_DGB) (src/impl/dgb/CMakeLists.txt:54),
so the pillar targets (dgb_share_test etc.) never existed and the build
failed with "No rule to make target dgb_share_test". Per the CMake
contract (dgb/CMakeLists.txt:17-20) COIN_DGB is required to build the
test targets; add it to the gate configure. No CMake/build.yml/coin-matrix
edit — PR #47 source guard untouched.

Author: frstrtr

tests/gates/dgb_phase_b_smoke.sh

@@ -27,7 +27,8 @@ if [ ! -f "$BUILD_DIR/CMakeCache.txt" ]; then
   cmake -S "$REPO_ROOT" -B "$BUILD_DIR" \
     -DCMAKE_TOOLCHAIN_FILE="$BUILD_DIR/conan_toolchain.cmake" \
     -DCMAKE_BUILD_TYPE=Release \
-    -DBUILD_TESTING=ON
+    -DBUILD_TESTING=ON \
+    -DCOIN_DGB=ON
 fi
 cmake --build "$BUILD_DIR" --target "${TARGETS[@]}" -j"$(nproc)"