flake.nix: add armv6l/armv7l systems

We built at least armv7l on hydra, therefor nixpkgs should also expose it.

flake.nix

@@ -12,7 +12,14 @@
 
       lib = import ./lib;
 
-      systems = [ "x86_64-linux" "i686-linux" "x86_64-darwin" "aarch64-linux" ];
+      systems = [
+        "x86_64-linux"
+        "i686-linux"
+        "x86_64-darwin"
+        "aarch64-linux"
+        "armv6l-linux"
+        "armv7l-linux"
+      ];
 
       forAllSystems = f: lib.genAttrs systems (system: f system);