Avoid partial functions Data.List.init and Data.List.last

[Bodigrim]

This is to comply with haskell/core-libraries-committee#292

We could have achieved the immediate goal by replacing combine in Distribution.Compat.Internal.TempFile with something like

    combine a [] = a
    combine a b = case unsnoc a of
        Nothing -> b
        Just (_, lastA)
            | pathSeparator [lastA] -> a ++ b
            | otherwise -> a ++ [pathSeparatorChar] ++ b

but honestly there is no reason to retain openNewBinaryFile implementation at all. Let's just outsource all the work to openBinaryTempFileWithDefaultPermissions from base.

Template B: This PR does not modify behaviour or interface

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• Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions).

[geekosaur]

Modulo the hlint warning, which derives from an import that is no longer conditional. —ETA: and which you just resolved while I was reviewing, lol.

[ulysses4ever]

Thanks!

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