Fedora default image config might not match official defaults

[achilleas-k]

While switching our CI runners to Fedora 40 images, built by IB itself, we immediately crashed and burned because of character encoding errors: https://gitlab.com/redhat/services/products/image-builder/ci/images/-/jobs/7027433444

Our default Fedora image config sets the locale to en_US.

images/pkg/distro/fedora/distro.go

Lines 440 to 444 in aad2c71

	var defaultDistroImageConfig = &distro.ImageConfig{
	Timezone: common.ToPtr("UTC"),
	Locale: common.ToPtr("en_US"),
	DefaultOSCAPDatastream: common.ToPtr(oscap.DefaultFedoraDatastream()),
	}

However, it seems that marketplace Fedora images use en_US.utf-8, which is why we never had this issue before.

We should update our image definitions to match.