check-meta.nix: make non-source consistent with documentation

The documentation for `meta.sourceProvenance` in
`doc/stdenv/meta.chapter.md` says: "the `meta.sourceProvenance`
attribute should be a list containing one or more value..."

Let's update check-meta.nix to require that `meta.sourceProvenance` is
a list, as the documentation says, rather than a single element.

Adding two extra keystrokes `[` and `]` when filling out this field is
an insignificant burden for package authors, and being able to assume
that the `meta.sourceProvenance` field is always a list greatly
simplifies any code that acts on the value of this field.

Since `meta.sourceProvenance` was just merged a few hours ago now is
the easiest time to fix this: nobody is using the feature yet.

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