lib.systems.supported: remove aarch64-darwin from Tier 3 list

While it is a fact of life that aarch64-darwin is built on Hydra, it has
never formally been elevated from the Tier 7 state it was originally
assigned in RFC 0046. Since platform Tier status is not only
descriptive, but also normative, a consensus to commit to supporting
aarch64-darwin would need to be reached.

lib/systems/supported.nix

···
 { lib }:
 rec {
   # List of systems that are built by Hydra.
-  hydra = tier1 ++ tier2 ++ tier3;
+  hydra = tier1 ++ tier2 ++ tier3 ++ [
+    "aarch64-darwin"
+  ];
 
   tier1 = [
     "x86_64-linux"
···
   ];
 
   tier3 = [
-    "aarch64-darwin"
     "armv6l-linux"
     "armv7l-linux"
     "i686-linux"