[CMake] Fix cmake error on already defined metis target

[olivier-roussel]

When cmake re-configuring an existing build, the configure was failing at the step of Sofa.Metis because the alias target metis::metis was already defined. Only tested this error with Pixi, but I think it should be fixed globally. Also, I could only reproduce the error locally on Windows, not on Linux, and I don't know why yet.

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