Deduplicate LD_LIBRARY_PATH when running tests (#8728)

* 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

---------

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

Author: 9999years

Cabal/src/Distribution/Simple/LocalBuildInfo.hs

@@ -295,7 +295,10 @@ depLibraryPaths inplace relative lbi clbi = do
               allDepLibDirsC
         | otherwise = allDepLibDirsC
 
-  return libPaths
+  -- For some reason, this function returns lots of duplicates. Avoid
+  -- exceeding `ARG_MAX` (the result of this function is used to populate
+  -- `LD_LIBRARY_PATH`) by deduplicating the list.
+  return $ ordNub libPaths
   where
     -- 'canonicalizePath' fails on UNIX when the directory does not exists.
     -- So just don't canonicalize when it doesn't exist.

changelog.d/pr-8728

@@ -0,0 +1,3 @@
+synopsis: Deduplicate LD_LIBRARY_PATH when running tests
+packages: Cabal
+prs: #8728