fix: build from sources with --no-binary=ninja
#276
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fix #274
Ninja shall never request ninja in its build dependencies as a default behavior.
When scikit-build-core knows a ninja binary wheel exists for a given platform, it's always added as a dependency when a valid one is not found on the system.
Let's allow ninja to depend on ninja only if explicitly allowed (this will speed-up the CI here). In other cases, it's more likely that a request to build ninja would be because of a forced source only build.