add .nix filter to feedback build

feedback.yaml

 loops:
-  build: cat .build-all.dhall | dhall text | bash
+  build:
+    command: cat .build-all.dhall | dhall text | bash
+    filter:
+      find: '*.nix'
   check: nix flake check -L
   gen:
     command: nix run .#horizon-gen-nix overlay