ci: Run relaxed appstream validation during tests

If appstream-util is installed the tests will run `appstream validate-relax` on
the metainfo file.

.gitlab-ci.yml

@@ -28,7 +28,7 @@ build:native:
     - export DEBIAN_FRONTEND=noninteractive
     - apt-get -y update
     - apt-get -y install eatmydata
-    - eatmydata apt-get -y install lcov
+    - eatmydata apt-get -y install lcov ca-certificates appstream-util
     - '[ "${WANT_BUILD_DEPS}" != "true" ] || eatmydata apt-get -y build-dep .'
     - ulimit -c unlimited
   script: