cuirass: Add job specs for the modular Guix.

* build-aux/cuirass/guix-modular.scm: New file.
* Makefile.am (EXTRA_DIST): Add it.

Makefile.am

@@ -469,6 +469,7 @@ EXTRA_DIST =						\
   build-aux/hydra/guix.scm				\
   build-aux/hydra/guix-modular.scm			\
   build-aux/cuirass/gnu-system.scm			\
+  build-aux/cuirass/guix-modular.scm			\
   build-aux/cuirass/hydra-to-cuirass.scm		\
   build-aux/check-available-binaries.scm		\
   build-aux/check-final-inputs-self-contained.scm	\

build-aux/cuirass/guix-modular.scm

@@ -0,0 +1,6 @@
+;;;
+;;; This file defines Cuirass build jobs to build Guix itself.
+;;;
+
+(include "../hydra/guix-modular.scm")
+(include "hydra-to-cuirass.scm")

build-aux/hydra/guix-modular.scm

@@ -86,15 +86,16 @@ for SYSTEM.  Use VERSION as the version identifier."
        (list (%current-system)))))
 
   (define guix-checkout
-    (assq-ref arguments 'guix))
+    (or (assq-ref arguments 'guix)                ;Hydra on hydra
+        (assq-ref arguments 'guix-modular)))      ;Cuirass on berlin
 
   (define version
     (or (assq-ref guix-checkout 'revision)
         "0.unknown"))
 
   (let ((file (assq-ref guix-checkout 'file-name)))
-    (format (current-error-port) "using checkout ~s (~s)~%"
-            guix-checkout file)
+    (format (current-error-port) "using checkout ~s (~s; arguments: ~s)~%"
+            guix-checkout file arguments)
 
     (map (lambda (system)
            (let ((name (string->symbol