Sort environment variables alphabetically in `derivation'.

* guix/derivations.scm (derivation)[env-vars-with-empty-outputs]: Sort
  the result alphabetically.

* tests/derivations.scm ("build derivation with 1 source"): Add
  environment variables.

guix/derivations.scm

@@ -308,19 +308,22 @@ known in advance, such as a file download."
 
   (define (env-vars-with-empty-outputs)
     ;; Return a variant of ENV-VARS where each OUTPUTS is associated with an
-    ;; empty string, even outputs that do not appear in ENV-VARS.
+    ;; empty string, even outputs that do not appear in ENV-VARS.  Note: the
+    ;; result is sorted alphabetically, as with C++ `std::map'.
     (let ((e (map (match-lambda
                    ((name . val)
                     (if (member name outputs)
                         (cons name "")
                         (cons name val))))
                   env-vars)))
-      (fold-right (lambda (output-name env-vars)
+      (sort (fold (lambda (output-name env-vars)
                     (if (assoc output-name env-vars)
                         env-vars
                         (append env-vars `((,output-name . "")))))
                   e
-                  outputs)))
+                  outputs)
+            (lambda (e1 e2)
+              (string<? (car e1) (car e2))))))
 
   (let* ((outputs    (map (lambda (name)
                             ;; Return outputs with an empty path.

tests/derivations.scm

@@ -59,7 +59,9 @@
                 ((drv-path drv)
                  (derivation %store "foo" "x86_64-linux"
                              "/bin/sh" `(,builder)
-                             '(("HOME" . "/homeless"))
+                             '(("HOME" . "/homeless")
+                               ("zzz"  . "Z!")
+                               ("AAA"  . "A!"))
                              `((,builder))))
                 ((succeeded?)
                  (build-derivations %store (list drv-path))))