Sort fingerprints when writing .sobelow-skips

[palm86]

Summary

Sobelow.mark_skip_all/1 writes the deduplicated set of finding
fingerprints to .sobelow-skips. The list comes from
Fingerprint.new_skips/0, which returns MapSet.to_list/1 — and
MapSet iteration order is implementation-defined, not stable.

Today the writer joins that list as-is:

fingerprints = Enum.join(fingerprints, "\n")

So running mix sobelow --mark-skip-all twice over the same set of
findings can produce different line orderings in .sobelow-skips,
which is noisy in code review and in git history when the file is
committed.

This change sorts the fingerprints before joining, making the on-disk
output deterministic for a given set of skipped findings:

fingerprints = fingerprints |> Enum.sort() |> Enum.join("\n")

Note on tests

There is no existing coverage for the .sobelow-skips write path. I
considered adding one but the function is defp and exercising it
end-to-end via the Mix task felt disproportionate to the size of this
fix. Happy to add coverage if you'd like to weigh in on the preferred
shape.

[houllette]

Hey @palm86 - thank you so much for the contribution! If you could, would you mind opening this as a PR on the new Sobelow repo? This PR belongs to the unmaintained version and it will no longer be receiving updates.