ci: Set LC_ALL=C to allow running the s390x tests in qemu

Without the fix, the tests immediately abort:

terminate called after throwing an instance of 'std::runtime_error'
  what():  locale::facet::_S_create_c_locale name not valid
./qt/test/test_bitcoin-qt: line 2: 116150 Aborted

ci/test/00_setup_env_s390x.sh

@@ -20,6 +20,7 @@ fi
 export CONTAINER_NAME=ci_s390x
 export DOCKER_NAME_TAG="debian:buster"
 export RUN_UNIT_TESTS=true
+export TEST_RUNNER_ENV="LC_ALL=C"
 export RUN_FUNCTIONAL_TESTS=true
 export GOAL="install"
 export BITCOIN_CONFIG="--enable-reduce-exports --with-incompatible-bdb --with-boost-process"

ci/test/06_script_b.sh

@@ -23,13 +23,13 @@ fi
 
 if [ "$RUN_UNIT_TESTS" = "true" ]; then
   BEGIN_FOLD unit-tests
-  DOCKER_EXEC DIR_UNIT_TEST_DATA=${DIR_UNIT_TEST_DATA} LD_LIBRARY_PATH=$DEPENDS_DIR/$HOST/lib make $MAKEJOBS check VERBOSE=1
+  DOCKER_EXEC ${TEST_RUNNER_ENV} DIR_UNIT_TEST_DATA=${DIR_UNIT_TEST_DATA} LD_LIBRARY_PATH=$DEPENDS_DIR/$HOST/lib make $MAKEJOBS check VERBOSE=1
   END_FOLD
 fi
 
 if [ "$RUN_UNIT_TESTS_SEQUENTIAL" = "true" ]; then
   BEGIN_FOLD unit-tests-seq
-  DOCKER_EXEC DIR_UNIT_TEST_DATA=${DIR_UNIT_TEST_DATA} LD_LIBRARY_PATH=$DEPENDS_DIR/$HOST/lib "${BASE_BUILD_DIR}/bitcoin-*/src/test/test_bitcoin*" --catch_system_errors=no -l test_suite
+  DOCKER_EXEC ${TEST_RUNNER_ENV} DIR_UNIT_TEST_DATA=${DIR_UNIT_TEST_DATA} LD_LIBRARY_PATH=$DEPENDS_DIR/$HOST/lib "${BASE_BUILD_DIR}/bitcoin-*/src/test/test_bitcoin*" --catch_system_errors=no -l test_suite
   END_FOLD
 fi