guix build: ‘--log-file’ honors the configured substitute URLs.

* guix/scripts/build.scm (guix-build): When defining ‘urls’, add
‘substitute-urls’ call.
* doc/guix.texi (Additional Build Options): Adjust ‘--log-file’
documentation.

Change-Id: I40f0647efe64ba3a63a6379b8da80b0da8910f48

doc/guix.texi

@@ -13714,7 +13714,7 @@ guix build --log-file -e '(@@ (gnu packages guile) guile-2.0)'
 
 If a log is unavailable locally, and unless @option{--no-substitutes} is
 passed, the command looks for a corresponding log on one of the
-substitute servers (as specified with @option{--substitute-urls}).
+substitute servers.
 
 So for instance, imagine you want to see the build log of GDB on
 @code{aarch64}, but you are actually on an @code{x86_64} machine:

guix/scripts/build.scm

@@ -763,9 +763,11 @@ needed."
                    (urls  (map (cut string-append <> "/log")
                                (if (assoc-ref opts 'substitutes?)
                                    (or (assoc-ref opts 'substitute-urls)
-                                       ;; XXX: This does not necessarily match the
+                                       (substitute-urls store)
-                                       ;; daemon's substitute URLs.
+                                       (begin
-                                       %default-substitute-urls)
+                                         (warning (G_ "could not determine current \
+substitute URLs; using defaults~%"))
+                                         %default-substitute-urls))
                                    '())))
                    (items (filter-map (match-lambda
                                         (('argument . (? store-path? file))