tests: Disable grafts in 'tests/builders.scm'.

This is required since the introduction of the OpenSSL graft in the
parent commit so that calling 'package-derivation' in the Python tests
would not trigger a build.

* tests/builders.scm <top level>: Add '%graft?' call.

tests/builders.scm

@@ -49,6 +49,9 @@
 (define url-fetch*
   (store-lower url-fetch))
 
+;; Globally disable grafts because they can trigger early builds.
+(%graft? #f)
+
 
 (test-begin "builders")