guix: consolidate back to GCC 12 toolchain for all HOSTS

Using GCC 11 for the macOS build hasn't been required since #21778, and
at this point, given a toolchain is still needed (#30206), it makes more
sense to (re-)use 12, rather than make all builders compile another
GCC toolchain.

Author: fanquake

contrib/guix/manifest.scm

@@ -500,6 +500,7 @@ inspecting signatures in Mach-O binaries.")
         gzip
         xz
         ;; Build tools
+        gcc-toolchain-12
         cmake-minimal
         gnu-make
         libtool
@@ -515,22 +516,16 @@ inspecting signatures in Mach-O binaries.")
         python-lief)
   (let ((target (getenv "HOST")))
     (cond ((string-suffix? "-mingw32" target)
-           (list ;; Native GCC 12 toolchain
-                 gcc-toolchain-12
-                 zip
+           (list zip
                  (make-mingw-pthreads-cross-toolchain "x86_64-w64-mingw32")
                  nsis-x86_64
                  nss-certs
                  osslsigncode))
           ((string-contains target "-linux-")
-           (list ;; Native GCC 12 toolchain
-                 gcc-toolchain-12
-                 (list gcc-toolchain-12 "static")
+           (list (list gcc-toolchain-12 "static")
                  (make-bitcoin-cross-toolchain target)))
           ((string-contains target "darwin")
-           (list ;; Native GCC 11 toolchain
-                 gcc-toolchain-11
-                 clang-toolchain-18
+           (list clang-toolchain-18
                  lld-18
                  (make-lld-wrapper lld-18 #:lld-as-ld? #t)
                  python-signapple