flake: fix attr-path of build-lowdown_2_0 job in hydra jobs

We would've had `build-lowdown_2_0.aarch64-linux.aarch64-linux`
otherwise. This is a bit of a problem because my way of generating a
list of constituents for Hydra's per-architecture[1] release job stops
recursing once it encounters a system.

The alternative would be to switch `hydraJobs` to `<system>.x.y` or
switching Hydra to "legacy" jobsets. For the latter I'd prefer to do the
same for Buildkite such that we don't have diverging things to build
depending on pre/post-merge CI.

[1] per-architecute because the trusted AFNix builders don't support all
    the architectures we support in Lix and with Flakes there's no way
    of parameterizing the list of supported systems.

Change-Id: I33f31260caf86ed5bb0f728770ca3cf1c00adf31

flake.nix

@@ -325,7 +325,7 @@
         build = forAllSystems (system: self.packages.${system}.nix);
 
         # Ensure support for lowdown < 3.0 doesn't regress for NixOS 25.11
-        build-lowdown_2_0.aarch64-linux = lib.genAttrs [ "aarch64-linux" ] (
+        build-lowdown_2_0 = lib.genAttrs [ "aarch64-linux" ] (
           system:
           self.packages.${system}.nix.override {
             lowdown = nixpkgsFor.${system}.native.lowdown;