Make valid-systems quicker

In the future, it would be good to pull this from the database again, but in
some way which is really quick.

guix-data-service/model/derivation.scm

@@ -59,10 +59,12 @@
             derivation-file-names->derivation-ids))
 
 (define (valid-systems conn)
-  (map car
-       (exec-query
-        conn
-        "SELECT DISTINCT system FROM derivations ORDER BY 1")))
+  ;; TODO, use the database, but make it quick!
+  '("aarch64-linux"
+    "armhf-linux"
+    "i686-linux"
+    "mips64el-linux"
+    "x86_64-linux"))
 
 (define (count-derivations conn)
   (first

tests/model-derivation.scm

@@ -9,7 +9,7 @@
  "test-model-derivation"
  (lambda (conn)
    (test-equal "valid-systems"
-     '()
+     '("aarch64-linux" "armhf-linux" "i686-linux" "mips64el-linux" "x86_64-linux")
      (valid-systems conn))
 
    (test-equal "count-derivations"