Declare nonguix as a channel dependency in .guix-channel

metznet/machines/otto.scm uses (nongnu packages mozilla), but guix pull
builds this channel's own modules in isolation unless a dependency is
declared, so that module came up as "no code for module" too. Declare
nonguix using the same introduction already used in channels.d/nonguix.scm.

.guix-channel

@@ -1,2 +1,15 @@
+;; metznet/machines/otto.scm uses (nongnu packages mozilla), so guix pull
+;; needs to know nonguix is a build-time dependency -- otherwise it tries
+;; to compile/load this channel's own modules in isolation and fails with
+;; "no code for module (nongnu packages mozilla))".
 (channel
-  (version 0))
+  (version 0)
+  (dependencies
+   (channel
+    (name nonguix)
+    (url "https://gitlab.com/nonguix/nonguix")
+    (introduction
+     (channel-introduction
+      (version 0)
+      (commit "897c1a470da759236cc11798f4e0a5f7d4d59fbc")
+      (signer "2A39 3FFF 68F4 EF7A 3D29 12AF 6F51 20A0 22FB B2D5"))))))