podvm-mkosi: Add support for Debian and Fedora variants

[BbolroC]

Since #1923 was merged, the CI job using podvm_mkosi_image.yaml has been failing.

It turns out docker buildx use default needs to be run in the GHA environment where the Ubuntu self-hosted runner is running.

This PR adds support for both Debian and Fedora variants.

Fixes: #1949

Signed-off-by: Hyounggyu Choi Hyounggyu.Choi@ibm.com

[stevenhorsman]

LGTM. Thanks @BbolroC

[BbolroC]

For helping reviewers understand what happened without docker buildx use default:

• A builder image fedora-binaries-builder-s390x is exported in the previous step named Build builder
• But in the following step (especially in a GHA context, this does not happen locally in the runner), docker buildx build tries to use another driver like building with "builder-8269f652-a1ec-423a-978e-b787fe00f975" instance using docker-container driver, but there the builder image is not stored for that driver

The absence of docker buildx use default leads to making it referring to docker.io/library/fedora-binaries-builder-s390x and the job fails like:

#2 [internal] load metadata for docker.io/library/fedora-binaries-builder-s390x:latest
#2 ERROR: pull access denied, repository does not exist or may require authorization: server message: insufficient_scope: authorization failed
------
 > [internal] load metadata for docker.io/library/fedora-binaries-builder-s390x:latest:
------

 1 warning found (use --debug to expand):
 - InvalidDefaultArgInFrom: Default value for ARG ${BUILDER_IMG} results in empty or invalid base image name (line 9)
Dockerfile.podvm_binaries.fedora:9
--------------------
   7 |     ARG BUILDER_IMG
   8 |     
   9 | >>> FROM ${BUILDER_IMG} AS podvm_builder
  10 |     
  11 |     ARG CLOUD_PROVIDER
--------------------
ERROR: failed to solve: fedora-binaries-builder-s390x: failed to resolve source metadata for docker.io/library/fedora-binaries-builder-s390x:latest: pull access denied, repository does not exist or may require authorization: server message: insufficient_scope: authorization failed

[bpradipt]

/lgtm