gitlab-ci: include full Rust backtraces in test runs

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

.gitlab-ci.d/buildtest-template.yml

@@ -63,6 +63,7 @@
   stage: test
   image: $CI_REGISTRY_IMAGE/qemu/$IMAGE:$QEMU_CI_CONTAINER_TAG
   script:
+    - export RUST_BACKTRACE=1
     - source scripts/ci/gitlab-ci-section
     - section_start buildenv "Setting up to run tests"
     - scripts/git-submodule.sh update roms/SLOF