nix/flake: fix MacOS build

The MacOS build fails with:

> ++ command cargo build --release --message-format json-render-diagnostics --locked
>    Compiling gitlab-timelogs v0.2.2 (/private/tmp/nix-build-gitlab-timelogs-0.2.2.drv-0/8bm81sjfqmsk7lclw7pnddrxv785aaj3-source)
> error: linking with `cc` failed: exit status: 1

Fix by manually adding iconv as a dependency.

Other people also have encountered this:
nix-community/home-manager#3482

nix/build.nix

@@ -3,6 +3,7 @@
 , lib
 , nix-gitignore
 , openssl
+, iconv
 , pkg-config
 , stdenv
 }:
@@ -17,9 +18,10 @@ let
     ];
     buildInputs = [
       openssl
-    ] ++ lib.optionals stdenv.isDarwin (with darwin.apple_sdk.frameworks; [
-      SystemConfiguration
-    ]);
+    ] ++ lib.optionals stdenv.isDarwin [
+      iconv
+      darwin.apple_sdk.frameworks.SystemConfiguration
+    ];
     # Fix build. Reference:
     # - https://github.com/sfackler/rust-openssl/issues/1430
     # - https://docs.rs/openssl/latest/openssl/