guix: Use clang-toolchain instead of clang

contrib/guix/manifest.scm

@@ -237,5 +237,5 @@ chain for " target " development."))
           ((string-contains target "-linux-")
            (list (make-bitcoin-cross-toolchain target)))
           ((string-contains target "darwin")
-           (list clang-8 binutils imagemagick libtiff librsvg font-tuffy cmake xorriso))
+           (list clang-toolchain-8 binutils imagemagick libtiff librsvg font-tuffy cmake xorriso))
           (else '())))))