nix-experimental: enable ca-derivations and impure-derivations

nixos/mixins/nix-experimental.nix

@@ -19,6 +19,16 @@
 
       # Allow derivation builders to call Nix, and thus build derivations recursively.
       "recursive-nix"
+
+      # Allow derivations to be content-addressed in order to prevent rebuilds
+      # when changes to the derivation do not result in changes to the
+      # derivation's output.
+      "ca-derivations"
+
+      # Allow derivations to produce non-fixed outputs by setting the __impure
+      # derivation attribute to true. An impure derivation can have differing
+      # outputs each time it is built.
+      "impure-derivations"
     ]
     ++ lib.optional (lib.versionAtLeast (lib.versions.majorMinor config.nix.package.version) "2.19")
       # Allow the use of the impure-env setting.