gnu: racket: suppress build stamp

The `build-stamp` configuration key should generally be set to an empty
string for a release build. The automatically inferred build stamp is
especially unhelpful with Guix, since it ends up being the Unix epoch.

* gnu/packages/racket.scm (racket-minimal)[arguments]: Adjust the
'initialize-config.rktd phase to set 'build-stamp to "".

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

gnu/packages/racket.scm

@@ -217,6 +217,7 @@
                                                    "/lib"))
                                   '("openssl"
                                     "sqlite"))))
+                    (build-stamp . "")
                     (catalogs
                      . (,(string-append
                           "https://download.racket-lang.org/releases/"