nixos/release-combined: adds aarch64-linux as supported

This was previously removed in 74c4e30842.

This will allow hydra to build iso and sd images for aarch64-linux, and
share a common channel with the x86-based platforms.

nixos/release-combined.nix

@@ -4,7 +4,7 @@
 
 { nixpkgs ? { outPath = (import ../lib).cleanSource ./..; revCount = 56789; shortRev = "gfedcba"; }
 , stableBranch ? false
-, supportedSystems ? [ "x86_64-linux" ]
+, supportedSystems ? [ "x86_64-linux" "aarch64-linux" ]
 , limitedSupportedSystems ? [ "i686-linux" ]
 }: