substitute-binary: Ignore $GUIX_BINARY_SUBSTITUTE_URL.

* guix/scripts/substitute-binary.scm (%cache-url): Ignore the
  'GUIX_BINARY_SUBSTITUTE_URL' environment variable.
* test-env.in: Invoke 'guix-daemon' with '--substitute-urls'.
* tests/substitute-binary.scm: Set '%cache-url' to the value of
  'GUIX_BINARY_SUBSTITUTE_URL'.

guix/scripts/substitute-binary.scm

@@ -625,20 +625,19 @@ found."
   (assoc-ref (daemon-options) option))
 
 (define %cache-url
-  (or (getenv "GUIX_BINARY_SUBSTITUTE_URL")
-      (match (and=> (find-daemon-option "substitute-urls")
-                    string-tokenize)
-        ((url)
-         url)
-        ((head tail ..1)
-         ;; Currently we don't handle multiple substitute URLs.
-         (warning (_ "these substitute URLs will not be used:~{ ~a~}~%")
-                  tail)
-         head)
-        (#f
-         ;; This can only happen when this script is not invoked by the
-         ;; daemon.
-         "http://hydra.gnu.org"))))
+  (match (and=> (find-daemon-option "substitute-urls")
+                string-tokenize)
+    ((url)
+     url)
+    ((head tail ..1)
+     ;; Currently we don't handle multiple substitute URLs.
+     (warning (_ "these substitute URLs will not be used:~{ ~a~}~%")
+              tail)
+     head)
+    (#f
+     ;; This can only happen when this script is not invoked by the
+     ;; daemon.
+     "http://hydra.gnu.org")))
 
 (define (guix-substitute-binary . args)
   "Implement the build daemon's substituter protocol."

test-env.in

@@ -81,7 +81,8 @@ then
     # Launch the daemon without chroot support because is may be
     # unavailable, for instance if we're not running as root.
     "@abs_top_builddir@/pre-inst-env"				\
-	"@abs_top_builddir@/guix-daemon" --disable-chroot &
+	"@abs_top_builddir@/guix-daemon" --disable-chroot	\
+	--substitute-urls="$GUIX_BINARY_SUBSTITUTE_URL" &
 
     daemon_pid=$!
     trap "kill $daemon_pid ; rm -rf $NIX_STATE_DIR" EXIT

tests/substitute-binary.scm

@@ -166,6 +166,9 @@ a file for NARINFO."
 (define-syntax-rule (with-narinfo narinfo body ...)
   (call-with-narinfo narinfo (lambda () body ...)))
 
+;; Transmit these options to 'guix substitute-binary'.
+(set! (@@ (guix scripts substitute-binary) %cache-url)
+      (getenv "GUIX_BINARY_SUBSTITUTE_URL"))
 
 (test-equal "query narinfo without signature"
   ""                                              ; not substitutable