nixos/release-combined.nix: makes aarch64-linux limited support

This is because it will not eval properly with `hydra-eval-jobs`.

```
$ ...hydra/result/bin/hydra-eval-jobs \
    --arg nixpkgs '{ outPath = ./.; revCount = 123; shortRev = "4567"; }' \
    -I "$PWD" \
    nixos/release-combined.nix
```

It fails with:

```
Too many heap sections: Increase MAXHINCR or MAX_HEAP_SECTS
```

nixos/release-combined.nix

@@ -4,8 +4,8 @@
 
 { nixpkgs ? { outPath = (import ../lib).cleanSource ./..; revCount = 56789; shortRev = "gfedcba"; }
 , stableBranch ? false
-, supportedSystems ? [ "x86_64-linux" "aarch64-linux" ]
-, limitedSupportedSystems ? [ "i686-linux" ]
+, supportedSystems ? [ "x86_64-linux" ]
+, limitedSupportedSystems ? [ "i686-linux" "aarch64-linux" ]
 }:
 
 let