Add missing pins and test ghc865 and ghc883 (#641)
Now that the defaultCompilerNixName is used to choose which GHC to use for tools we need to update haskellNixRoots to make sure the common tools wind up in the cache.
To do this we add the compilerNixName as a dimension in ci.nix. This has the added bonus of ensuring the tests a run with the supported versions of ghc (not just the default one).
haskellNixRoots is also updated to include the tools built with the defaultCompilerNixName (in particular cabal-install was not pinned and this was the likely cause of significant builds due to cache misses).