Merge bitcoin/bitcoin#23845: ci: use GCC 8 when building packages in native_qt5 CI

2da97b2 ci: use GCC 8 when building packages in native_qt5 CI (fanquake)

Pull request description:

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

  This should have been included as part of 182de7b.

ACKs for top commit:
  MarcoFalke:
    cr ACK 2da97b2
  hebasto:
    ACK 2da97b2

Tree-SHA512: 2b64c21119fb95b959ac0f7d8d2d38f6ed98309695bb35425fad45b3b628247c2c78d0647c4d1f511669e8d333c6febe1cc44fac8027ed0bd7593eb630add548

Author: MarcoFalke

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"