Building a bootc `anaconda-iso` from Fedora fails with unset vendor

[supakeen]

When doing:

sudo image-builder build --bootc-ref quay.io/fedora/fedora-bootc:rawhide --bootc-default-fs ext4 anaconda-iso

We seem to not set the vendor for the org.osbuild.grub2.iso stage causing the build to fail with:

Traceback (most recent call last):
File "/run/osbuild/bin/org.osbuild.grub2.iso", line 136, in <module>
    ret = main(args["tree"], args["options"])
  File "/run/osbuild/bin/org.osbuild.grub2.iso", line 70, in main
    vendor = options["vendor"]
             ~~~~~~~^^^^^^^^^^
KeyError: 'vendor'