tests: Fix typo in 'tests/store.scm'.

This typo had always been there since the test was introduced in commit
ce72c78074.  Presumably, it became visible
with 9e5812ac59, where caching was no
longer global and thus the 'store' argument of 'package-derivation' was
actually being used.

* tests/store.scm ("current-build-output-port, UTF-8"): Refer to
'%store' rather than 's' in 'package-derivation' call.

tests/store.scm

@@ -534,7 +534,8 @@
                  (d (build-expression->derivation
                      %store "foo" `(display ,s)
                      #:guile-for-build
-                     (package-derivation s %bootstrap-guile (%current-system)))))
+                     (package-derivation %store %bootstrap-guile
+                                         (%current-system)))))
             (guard (c ((store-protocol-error? c) #t))
               (build-derivations %store (list d))))))))
    "Here’s a Greek letter: λ."))