tests: Run without substitutes.

* tests/builders.scm, tests/derivations.scm, tests/packages.scm: Set
  #:use-substitutes? #f for %STORE.

tests/builders.scm

@@ -34,6 +34,10 @@
 (define %store
   (false-if-exception (open-connection)))
 
+(when %store
+  ;; Make sure we build everything by ourselves.
+  (set-build-options %store #:use-substitutes? #f))
+
 (define %bootstrap-inputs
   ;; Use the bootstrap inputs so it doesn't take ages to run these tests.
   ;; This still involves building Make, Diffutils, and Findutils.