Add a Hydra job to track the size of the initrd

nixos/release.nix

···
   manpages = buildFromConfig ({ pkgs, ... }: { }) (config: config.system.build.manual.manpages);
 
 
+  # Build the initial ramdisk so Hydra can keep track of its size over time.
+  initialRamdisk = buildFromConfig ({ pkgs, ... }: { }) (config: config.system.build.initialRamdisk);
+
+
   iso_minimal = forAllSystems (system: makeIso {
     module = ./modules/installer/cd-dvd/installation-cd-minimal.nix;
     type = "minimal";