fix: error on target platform != x86_64-unknown-linux-gnu

[haenoe]

Hey there, thanks a lot for maintaining this! Hope you don't mind me skipping the issue step.

When trying to use this flake on Darwin, I ran into the following error:

❯ nix run github:typst/typst-flake
error:
       … while evaluating the attribute 'value'
         at /nix/store/2f0w7jys97fs8sld7qqg1ck7h2j3asjg-source/lib/modules.nix:1118:7:
         1117|     // {
         1118|       value = addErrorContext "while evaluating the option `${showOption loc}':" value;
             |       ^
         1119|       inherit (res.defsFinal') highestPrio;

       … while calling the 'addErrorContext' builtin
         at /nix/store/2f0w7jys97fs8sld7qqg1ck7h2j3asjg-source/lib/modules.nix:1118:15:
         1117|     // {
         1118|       value = addErrorContext "while evaluating the option `${showOption loc}':" value;
             |               ^
         1119|       inherit (res.defsFinal') highestPrio;

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

       error: The `env` attribute set can only contain derivation, string, boolean or integer attributes. The `RUSTFLAGS` attribute is of type null.

This is due the fact that RUSTFLAGS is set to null, when target != x86_64-unknown-linux-gnu see here

rustflags =
            if pkgs.stdenv.hostPlatform.rust.rustcTargetSpec == "x86_64-unknown-linux-gnu" then
              # Upstream defaults to lld on x86_64-unknown-linux-gnu, we need to use the system linker
              "-Clinker-features=-lld -Clink-self-contained=-linker"
            else
              null;

Relevant check in nixpkgs

        mapAttrs (
          n: v:
          assert assertMsg (isString v || isBool v || isInt v || isDerivation v)
            "The `env` attribute set can only contain derivation, string, boolean or integer attributes. The `${n}` attribute is of type ${builtins.typeOf v}.";
          v
        ) env';

Minor note: as per https://doc.rust-lang.org/cargo/reference/config.html#buildrustflags an empty RUSTFLAGS still overrides user configuration (e.g. in cargo config); see this