* Deduplicate `LD_LIBRARY_PATH` when running tests

When Cabal runs a test suite, it adds all the dependent libraries to
`LD_LIBRARY_PATH`. In some cases, this can exceed the operating system's
`ARG_MAX` limit:

    dist/build/test/test: createProcess: posix_spawnp: resource exhausted (Argument list too long)

However, many of the entries (observed as high as 75%!) in `LD_LIBRARY_PATH`
are duplicates. Deduplicating the list with `Data.List.nub` fixes this error.

This error was originally observed when testing a large commercial
Haskell project (the mercury.com backend). Ideally, `depLibraryPaths`
would be fixed to not return duplicate entries, but this fixup is
low-cost and effective.

* Add changelog entries

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Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>