build-system/guile: Install .scm files first.

Until now the .go files were generated first, and only after that the .scm
files were installed into the target location.  That led to a lot of messages
about `source file ... newer than compiled' if the custom 'check phase tried
to load the compiled files.

Swapping the order of the actions resolves the issue allowing the tests to be
written without lot of noise in the build log.

For final artifacts it was not a problem, since daemon resets the timestamps.

* guix/build/guile-build-system.scm (build): Install .scm before producing
.go.

Change-Id: I3428d144fcbaa6c904ee662193c3bca82589e344
Signed-off-by: Ludovic Courtès <ludo@gnu.org>

guix/build/guile-build-system.scm

@@ -187,6 +187,12 @@ (define* (build #:key outputs inputs native-inputs
     (let ((source-files
            (with-directory-excursion source-directory
              (find-files "." scheme-file-regexp))))
+      (for-each
+       (lambda (file)
+         (install-file (string-append source-directory "/" file)
+                       (string-append module-dir
+                                      "/" (dirname file))))
+       source-files)
       (invoke-each
        (filter-map (lambda (file)
                      (and (or (not not-compiled-file-regexp)
@@ -202,14 +208,7 @@ (define* (build #:key outputs inputs native-inputs
                                  flags)))
                    source-files)
        #:max-processes (parallel-job-count)
-       #:report-progress report-build-progress)
-
-      (for-each
-       (lambda (file)
-         (install-file (string-append source-directory "/" file)
-                       (string-append module-dir
-                                      "/" (dirname file))))
-       source-files))
+       #:report-progress report-build-progress))
     #t))
 
 (define* (install-documentation #:key outputs