osbuild: use sparse for GCP tar artifact

The support for this was added to the tar stage upstream in [1]
and we just never applied it to the manifests here.

[1] osbuild/osbuild#1614

Author: dustymabe

src/osbuild-manifests/platform.gcp.ipp.yaml

@@ -68,6 +68,7 @@ pipelines:
           filename: disk.tar
           format: oldgnu
           root-node: omit
+          sparse: true
           # Set these to false so GCP image upload/create will succeed
           acls: false
           selinux: false