radish: add borgbackup

hosts/radish/secrets.nix

@@ -0,0 +1,9 @@
+{ lib, ... }:
+{
+  age.secrets = lib.listToAttrs
+    (
+      map (secret: { name = secret; value = { file = ../../secrets/${secret}.age; }; }) [
+        "borgbackup-radish"
+      ]
+    );
+}