ci: Install needed gcc and qemu-user iff cross-compiling

ci/test/00_setup_env_s390x.sh

@@ -10,7 +10,12 @@ export HOST=s390x-linux-gnu
 # The host arch is unknown, so we run the tests through qemu.
 # If the host is s390x and wants to run the tests natively, it can set QEMU_USER_CMD to the empty string.
 if [ -z ${QEMU_USER_CMD+x} ]; then export QEMU_USER_CMD="${QEMU_USER_CMD:-"qemu-s390x"}"; fi
-export PACKAGES="python3-zmq bsdmainutils qemu-user"
+export PACKAGES="python3-zmq"
+if [ -n "$QEMU_USER_CMD" ]; then
+  # Likely cross-compiling, so install the needed gcc and qemu-user
+  export DPKG_ADD_ARCH="s390x"
+  export PACKAGES="$PACKAGES g++-s390x-linux-gnu qemu-user libc6:s390x libstdc++6:s390x libfontconfig1:s390x libxcb1:s390x"
+fi
 export RUN_UNIT_TESTS=true
 export RUN_FUNCTIONAL_TESTS=true
 export GOAL="install"