Switch ghcjs builds to nixpkgs-unstable for Big Sur

Author: hamishmack

ci.nix

@@ -43,7 +43,7 @@
     # We need to use the actual nixpkgs version we're working with here, since the values
     # of 'lib.systems.examples' are not understood between all versions
     let lib = nixpkgs.lib;
-    in lib.optionalAttrs (nixpkgsName == "R2009" && (__elem compiler-nix-name ["ghc865" "ghc884"])) {
+    in lib.optionalAttrs (nixpkgsName == "unstable" && (__elem compiler-nix-name ["ghc865" "ghc884"])) {
     inherit (lib.systems.examples) ghcjs;
   } // lib.optionalAttrs (system == "x86_64-linux" && (
          (nixpkgsName == "R2009" && __elem compiler-nix-name ["ghc8101" "ghc8102" "ghc8103" "ghc8104" "ghc810220201118"])

release.nix

@@ -1,7 +1,7 @@
 # 'supportedSystems' restricts the set of systems that we will evaluate for. Useful when you're evaluating
 # on a machine with e.g. no way to build the Darwin IFDs you need!
 { supportedSystems ? [ "x86_64-linux" "x86_64-darwin" ]
-, ifdLevel ? 3
+, ifdLevel ? 2
 , checkMaterialization ? false }:
 
 let