ci: use GCC 8 when building packages in native_qt5 CI

Our minimum required GCC is GCC 8, and this change in required for
changes like #23839 which take advantage of flags introduced in that
version of GCC.

This should have been included as part of
182de7b.

Author: fanquake

ci/test/00_setup_env_native_qt5.sh

@@ -9,7 +9,7 @@ export LC_ALL=C.UTF-8
 export CONTAINER_NAME=ci_native_qt5
 export DOCKER_NAME_TAG=ubuntu:18.04  # Check that bionic gcc-8 can compile our C++17 and run our functional tests in python3, see doc/dependencies.md
 export PACKAGES="gcc-8 g++-8 python3-zmq qtbase5-dev qttools5-dev-tools libdbus-1-dev libharfbuzz-dev"
-export DEP_OPTS="NO_QT=1 NO_UPNP=1 NO_NATPMP=1 DEBUG=1 ALLOW_HOST_PACKAGES=1"
+export DEP_OPTS="NO_QT=1 NO_UPNP=1 NO_NATPMP=1 DEBUG=1 ALLOW_HOST_PACKAGES=1 CC=gcc-8 CXX=g++-8"
 export TEST_RUNNER_EXTRA="--previous-releases --coverage --extended --exclude feature_dbcrash"  # Run extended tests so that coverage does not fail, but exclude the very slow dbcrash
 export RUN_UNIT_TESTS_SEQUENTIAL="true"
 export RUN_UNIT_TESTS="false"