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.
607d59fa9e