osbuild: Rename GCE output from .tgz to .tar.gz

[pastequo]

Import with gcloud cli requires the file to ends with .tar.gz, otherwise the command fails

gcloud compute images create --help
[...]
       --source-uri=SOURCE_URI
          The full Cloud Storage URI where the disk image is stored. This file
          must be a gzip-compressed tarball whose name ends in .tar.gz. For
          more information about Cloud Storage URIs, see
          https://cloud.google.com/storage/docs/request-endpoints#json-api.

[pastequo]

Also updated the test in bib: osbuild/bootc-image-builder#679

[mvo5]

Thank you! I And sorry that I missed this.