nixos/repart: add loop module to initrd

Systemd-repart will use loopback devices for partition creation if it is
able to, and will fallback to doing "offline" partition creation writing
data directly to files. From what I see looking at the repart code,
there are specific features that cannot be taken advantage of when not
using loopback devices (e.g. no BTRFS subvolumes in systemd v255) and in
certain places they have to perform some manual re-sizing work that can
otherwise be avoided.

nixos/modules/system/boot/systemd/repart.nix

@@ -83,6 +83,9 @@ in
       }
     ];
 
+    # systemd-repart uses loopback devices for partition creation
+    boot.initrd.availableKernelModules = lib.optional initrdCfg.enable "loop";
+
     boot.initrd.systemd = lib.mkIf initrdCfg.enable {
       additionalUpstreamUnits = [
         "systemd-repart.service"