Fix native compilation on aarch64

Turns out fenix.packages.aarch64-linux.targets.x86_64-*.minimal.rustc
is an x86_64 compiler. The only component that you actually need to pull
from 'targets' for cross compilation is rust-std.

Because x86_64-unknown-linux-gnu is first in the target list, we were
getting x86 rustc and cargo binaries, making it impossible to compile
grapevine on an aarch64 host.

We were also missing aarch64-unknown-linux-gnu in the targets list,
which is used by the default package on a aarch64 host.

flake.nix

@@ -59,12 +59,16 @@
             toolchainFile.toolchain.components;
           targets = toolchainFile.toolchain.targets;
           fenix = inputs.fenix.packages.${pkgs.stdenv.buildPlatform.system};
-        in
-          fenix.combine (builtins.map
+
+          nativeToolchain = (fenix.fromManifestFile inputs.rust-manifest)
+            .withComponents components;
+          crossComponents = builtins.map
             (target:
               (fenix.targets.${target}.fromManifestFile inputs.rust-manifest)
-              .withComponents components)
-            targets);
+                .rust-std)
+            targets;
+        in
+          fenix.combine ([nativeToolchain] ++ crossComponents);
 
         website-root = self.callPackage ./nix/pkgs/website-root {};
       });

rust-toolchain.toml

@@ -17,5 +17,6 @@ components = [
 targets = [
     "x86_64-unknown-linux-gnu",
     "x86_64-unknown-linux-musl",
+    "aarch64-unknown-linux-gnu",
     "aarch64-unknown-linux-musl",
 ]