lib.systems: remove supported, replace with flakeExposed

Since the list only gates the platforms the nixpkgs flake exposes
packages to build on, the `hydra` label made little sense. It was also
only used for this purpose, so the `tier*` attributes were largely
unnecessary.

To reflect the intention more accurately, we expose
`lib.systems.flakeExposed` and use it to gate flake.nix's system list.

flake.nix

@@ -11,9 +11,7 @@
 
       lib = import ./lib;
 
-      systems = lib.systems.supported.hydra;
-
-      forAllSystems = f: lib.genAttrs systems (system: f system);
+      forAllSystems = f: lib.genAttrs lib.systems.flakeExposed (system: f system);
 
     in
     {