Fix $BUILD_VERSION not being passed to build-inside-vm.sh

Fixes: 3bda5b26 ("Use new experimental VM runners[1] for building")

.gitlab-ci.yml

@@ -37,7 +37,7 @@ shfmt:
         echo "BUILD_VERSION=$(date +%Y%m%d).$CI_JOB_ID" > build.env
       fi
     - export $(< build.env)
-    - ./build-inside-vm.sh
+    - ./build-inside-vm.sh "${BUILD_VERSION}"
   after_script:
     - echo "image_size_megabytes{image=\"basic\"} $(du -m output/*basic*qcow2)" > metrics.txt
     - echo "image_size_megabytes{image=\"cloudimg\"} $(du -m output/*cloudimg*qcow2)" > metrics.txt