ci: Fix QEMU_USER_CMD parse issues

* Fix bash syntax by adding missing -

* Disable QEMU_USER_CMD fallback when it is set

  Apparently bash can't distinguish an unset variable from a variable
  that is set to the empty string

* Export the environment variable to the docker env, otherwise it
  couldn't be used there

ci/test/00_setup_env_s390x.sh

@@ -9,7 +9,7 @@ export LC_ALL=C.UTF-8
 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.
-export QEMU_USER_CMD="${QEMU_USER_CMD:"qemu-s390x"}"
+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 RUN_UNIT_TESTS=true
 export RUN_FUNCTIONAL_TESTS=true