guix: use GCC 12.4.0 over 12.3.0

Our patch might be merged upstream soon:
https://lists.gnu.org/archive/html/guix-patches/2024-06/msg01025.html.

In the mean time, it's easy us for us to use the newer version of GCC.

contrib/guix/manifest.scm

@@ -25,6 +25,7 @@
              (guix build-system gnu)
              (guix build-system python)
              (guix build-system trivial)
+             (guix download)
              (guix gexp)
              (guix git-download)
              ((guix licenses) #:prefix license:)
@@ -91,7 +92,18 @@ chain for " target " development."))
       (home-page (package-home-page xgcc))
       (license (package-license xgcc)))))
 
-(define base-gcc gcc-12)
+(define base-gcc
+  (package
+    (inherit gcc-12) ;; 12.3.0
+    (version "12.4.0")
+    (source (origin
+              (method url-fetch)
+              (uri (string-append "mirror://gnu/gcc/gcc-"
+                                  version "/gcc-" version ".tar.xz"))
+              (sha256
+               (base32
+                "0xcida8l2wykvvzvpcrcn649gj0ijn64gwxbplacpg6c0hk6akvh"))))))
+
 (define base-linux-kernel-headers linux-libre-headers-6.1)
 
 (define* (make-bitcoin-cross-toolchain target