podvm_builder: Add s390x gh install support

[stevenhorsman]

#2074 introduced the installation of the gh cli into the fedora podvm_builder Dockerfile using dnf. This isn't available for s390x and there isn't a released s390x version of the gh cli, so build it from source on that architecture.

[stevenhorsman]

Tested on s390x:

cloud-api-adaptor/src/cloud-api-adaptor/podvm-mkosi# make fedora-binaries-builder
...
 => => naming to docker.io/library/fedora-binaries-builder-s390x
cloud-api-adaptor/src/cloud-api-adaptor/podvm-mkosi# docker run --rm -it fedora-binaries-builder-s390x bash -c "gh version"
gh version 2.60.1-12-g14d339d9 (2024-10-30)
cloud-api-adaptor/src/cloud-api-adaptor/podvm-mkosi# make binaries
...
#10 exporting to client directory
#10 copying files 24.27MB 0.1s
#10 copying files 236.73MB 1.0s done
#10 DONE 1.0s

Tested on x86 for regression purposes:

root@sh-mkosi-build1:~/go/src/github.com/confidential-containers/cloud-api-adaptor/src/cloud-api-adaptor/podvm-mkosi# make fedora-binaries-builder
Building fedora-binaries-builder-amd64 image...
...
 => => naming to docker.io/library/fedora-binaries-builder-amd64                                                                                                         0.0s
root@sh-mkosi-build1:~/go/src/github.com/confidential-containers/cloud-api-adaptor/src/cloud-api-adaptor/podvm-mkosi# docker run --rm -it fedora-binaries-builder-amd64 bash -c "gh version"
gh version 2.60.1 (2024-10-25)
https://github.com/cli/cli/releases/tag/v2.60.1
root@sh-mkosi-build1:~/go/src/github.com/confidential-containers/cloud-api-adaptor/src/cloud-api-adaptor/podvm-mkosi# make binaries
...
#10 exporting to client directory
#10 copying files 29B
#10 copying files 209.58MB 1.6s done
#10 DONE 1.6s

[BbolroC]

LGTM as far as the CI goes green. Thanks @stevenhorsman !