propagation: don't count propagated variant values as non-default (#33687)

We try to avoid non-default variant values in the concretizer, but this doesn't make
sense for variants forced to take some non-default value by variant propagation.
Counting this as a penalty effectively biases the concretizer for small specs dependency
graphs -- something we try very hard to avoid elsewhere because it can lead to very
strange decisions.

Example: with the penalty, `spack spec hdf5` will choose the default `openmpi` as its
`mpi` provider, but `spack spec hdf5 ~~shared` will choose `mpich` because it has to set
fewer non-default variant values because `mpich`'s DAG is smaller. That's not a good
reason to prefer a non-default virtual provider.

To fix this, if the user explicitly requests a non-default value to be propagated, there
shouldn't be a penalty. Variant values set on the CLI already don't count as default; we
just need to extend that to propagated values.

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