ghcup metadata includes extraneous dlOutput fields

ghcup can apparently infer the output name of an artifact from its URL. Consequently, we should only include the dlOutput field when it would differ from the filename of dlUri, as requested in https://github.com/haskell/ghcup-metadata/pull/186/files#diff-b7aa200da91425001d95ee963dc2b97e33c87f584d916569c69fa20b49795887.

Edited Mar 14, 2024 by Ben Gamari

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