message: guix: Use gcc-8 for mingw-w64 instead of 7
  We're using mingw-w64 6.0.0, which is paired with gcc-8 in most distros.
diff: @@ -114,7 +114,7 @@ desirable for building Bitcoin Core release binaries."
  (pthreads-xlibc mingw-w64-x86_64-winpthreads)
  (pthreads-xgcc (make-gcc-with-pthreads
  (cross-gcc target
  - #:xgcc (make-ssp-fixed-gcc gcc)
  + #:xgcc (make-ssp-fixed-gcc gcc-8)
  #:xbinutils xbinutils
  #:libc pthreads-xlibc))))
  ;; Define a meta-package that propagates the resulting XBINUTILS, XLIBC, and
  