Clean up packaging a bit

- Multiple choices of stdenv are handled more consistently, especially for the dev
  shells which were previously not done correctly.

- Some stray nix code was moving into the `packaging` directory

scripts/installer.nix

@@ -1,36 +0,0 @@
-{ lib
-, runCommand
-, nix
-, tarballs
-}:
-
-runCommand "installer-script" {
-  buildInputs = [ nix ];
-} ''
-  mkdir -p $out/nix-support
-
-  # Converts /nix/store/50p3qk8k...-nix-2.4pre20201102_550e11f/bin/nix to 50p3qk8k.../bin/nix.
-  tarballPath() {
-    # Remove the store prefix
-    local path=''${1#${builtins.storeDir}/}
-    # Get the path relative to the derivation root
-    local rest=''${path#*/}
-    # Get the derivation hash
-    local drvHash=''${path%%-*}
-    echo "$drvHash/$rest"
-  }
-
-  substitute ${./install.in} $out/install \
-    ${lib.concatMapStrings
-      (tarball: let
-          inherit (tarball.stdenv.hostPlatform) system;
-        in '' \
-        --replace '@tarballHash_${system}@' $(nix --experimental-features nix-command hash-file --base16 --type sha256 ${tarball}/*.tar.xz) \
-        --replace '@tarballPath_${system}@' $(tarballPath ${tarball}/*.tar.xz) \
-        ''
-      )
-      tarballs
-    } --replace '@nixVersion@' ${nix.version}
-
-  echo "file installer $out/install" >> $out/nix-support/hydra-build-products
-''