.github: Add Flatcar extensions to vm-images artifact

The Flatcar extensions get built by the GitHub PR CI but only their
content files get archived. Add the .raw image itself so that one can
copy it into the image (downloading it at boot time won't work because
this uses bincache - so one could get an extension image in case the but
version happens to match but it won't be the one that was built in the
GitHub CI).

.github/workflows/ci.yaml

@@ -319,6 +319,7 @@ jobs:
             scripts/artifacts/images/*.bin
             scripts/artifacts/images/flatcar_production_*_efi_*.fd
             scripts/artifacts/images/*.txt
+            scripts/artifacts/images/flatcar-*.raw
             scripts/artifacts/images/flatcar_production_*.sh
             scripts/artifacts/images/flatcar_production_pxe_image.cpio.gz
             scripts/artifacts/images/flatcar_production_pxe.vmlinuz