make-container-image: sleep only after full mediawiki image build
As a workaround for T390251, we inserted a 5m pause at the end of the build / push process.
As a modest improvement, we can limit this to when we are bitten by eventual consistency most often: after a full mediawiki image build.
To do so, this shifts the sleep up into build_mediawiki_image, where it's easier to access the per-flavour report indicating build_type.
That does mean that the sleep happens on a per-flavour basis (i.e., in parallel across flavours), but I don't think that's really a meaningful distinction (e.g., worth plumbing this out into main and sleeping after thread join).
Bug: T390251