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

@@ -625,7 +625,6 @@ (define (find-daemon-option option)
   (assoc-ref (daemon-options) option))
 
 (define %cache-url
-  (or (getenv "GUIX_BINARY_SUBSTITUTE_URL")
   (match (and=> (find-daemon-option "substitute-urls")
                 string-tokenize)
     ((url)
@@ -638,7 +637,7 @@ (define %cache-url
     (#f
      ;; This can only happen when this script is not invoked by the
      ;; daemon.
-         "http://hydra.gnu.org"))))
+     "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 @@ (define (call-with-narinfo narinfo thunk)
 (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