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.

Author: dongcarl

contrib/guix/manifest.scm

@@ -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