haskell.compiler: allow overriding source with hadrian

Hadrian (the GHC build tool) is built separately from GHC. This means
that if `haskell.compiler.ghc961` is overridden to add patches, those
patches will _only_ be applied to the GHC portion of the build, and not
the Hadrian build. For example, backporting this patch to GHC 9.6.1
failed because the changes to `hadrian/` files were not reflected in the
Nix build:

https://gitlab.haskell.org/ghc/ghc/-/commit/5ed77deb1b49bff7bae9660487ee4a40a496476d

By lifting `src` and `hadrian` from variables defined in the function
body to parameters with default values, the `hadrian/` files can be
overridden using the `haskell.compiler.ghc961.override` function. For
example:

   self.haskell.compiler.ghc961.override {
     # The GHC 9.6 builder in nixpkgs first builds hadrian with the
     # source tree provided here and then uses the built hadrian to
     # build the rest of GHC. We need to make sure our patches get
     # included in this `src`, then, rather than modifying the tree in
     # the `patchPhase` or `postPatch` of the outer builder.
     src = self.applyPatches {
       src = let
         version = "9.6.1";
       in
         self.fetchurl {
           url = "https://downloads.haskell.org/ghc/${version}/ghc-${version}-src.tar.xz";
           sha256 = "fe5ac909cb8bb087e235de97fa63aff47a8ae650efaa37a2140f4780e21f34cb";
         };

       patches = [
         # Enable response files for linker if supported
         (self.fetchpatch {
           url = "https://gitlab.haskell.org/ghc/ghc/-/commit/5ed77deb1b49bff7bae9660487ee4a40a496476d.patch";
           hash = "sha256-dvenK+EPTZJYXnyfKPdkvLp+zeUmsY9YrWpcGCzYStM=";
         })
       ];
     };
   }

Note that we do have to re-declare the `src` we want, but I'm not sure
of a good way to avoid this while also sharing one set of patches
between the GHC and Hadrian builds.

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