flake: Remove darwin CoreFoundation dev-shell input

The dev-shell failed since the recent nixpkgs updates like so:

       … while calling the 'derivationStrict' builtin
         at <nix/derivation-internal.nix>:37:12:
           36|
           37|   strict = derivationStrict drvAttrs;
             |            ^
           38|

       … while evaluating derivation 'nix-shell'
         whose name attribute is located at /nix/store/9v6qa656sq3xc58vkxslqy646p0ajj61-source/pkgs/stdenv/generic/make-derivation.nix:544:13

       … while evaluating attribute '__impureHostDeps' of derivation 'nix-shell'
         at /nix/store/9v6qa656sq3xc58vkxslqy646p0ajj61-source/pkgs/stdenv/generic/make-derivation.nix:694:15:
          693|               );
          694|               __impureHostDeps =
             |               ^
          695|                 computedImpureHostDeps

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)

       error: darwin.apple_sdk_11_0 has been removed as it was a legacy compatibility stub; see <https://nixos.org/manual/nixpkgs/stable/#sec-darwin-legacy-frameworks> for migration instructions

According to the link, the CoreFoundation input has been a stub for a
while and we can remove it.

Author: the-mikedavis

flake.nix

@@ -71,8 +71,7 @@
                 rust-bin.nightly.latest.rust-analyzer
               ]
               ++ (lib.optional (stdenv.isx86_64 && stdenv.isLinux) cargo-tarpaulin)
-              ++ (lib.optional stdenv.isLinux lldb)
-              ++ (lib.optional stdenv.isDarwin darwin.apple_sdk.frameworks.CoreFoundation);
+              ++ (lib.optional stdenv.isLinux lldb);
             shellHook = ''
               export RUST_BACKTRACE="1"
               export RUSTFLAGS="''${RUSTFLAGS:-""} ${commonRustFlagsEnv} ${platformRustFlagsEnv}"