store: 'run-with-store' initializes %CURRENT-TARGET-SYSTEM to #f.

* guix/store.scm (run-with-store): Set %CURRENT-TARGET-SYSTEM to #f.
* tests/gexp.scm ("gexp->derivation vs. %current-target-system"): New test.

guix/store.scm

@@ -996,8 +996,12 @@ permission bits are kept."
                          (system (%current-system)))
   "Run MVAL, a monadic value in the store monad, in STORE, an open store
 connection, and return the result."
+  ;; Initialize the dynamic bindings here to avoid bad surprises.  The
+  ;; difficulty lies in the fact that dynamic bindings are resolved at
+  ;; bind-time and not at call time, which can be disconcerting.
   (parameterize ((%guile-for-build guile-for-build)
-                 (%current-system system))
+                 (%current-system system)
+                 (%current-target-system #f))
     (call-with-values (lambda ()
                         (run-with-state mval store))
       (lambda (result store)

tests/gexp.scm

@@ -636,6 +636,17 @@
                                             file)))))
       #:guile-for-build (package-derivation %store %bootstrap-guile))))
 
+(test-assert "gexp->derivation vs. %current-target-system"
+  (let ((mval (gexp->derivation "foo"
+                                #~(begin
+                                    (mkdir #$output)
+                                    (foo #+gnu-make))
+                                #:target #f)))
+    ;; The value of %CURRENT-TARGET-SYSTEM at bind-time should have no
+    ;; influence.
+    (parameterize ((%current-target-system "fooooo"))
+      (derivation? (run-with-store %store mval)))))
+
 (test-assert "printer"
   (string-match "^#<gexp \\(string-append .*#<package coreutils.*\
  \"/bin/uname\"\\) [[:xdigit:]]+>$"