Force copying files for mruby

When installing mruby-dev, the cp command doesn't use the force flag.
This causes an issue when using a custom build configuration (via
MRUBY_CONFIG environment variable) and installing mrbgems from Github,
as the pack files in the git repositories are read-only.

bin/ruby-build

@@ -449,7 +449,7 @@ build_package_mruby() {
 
   { rake
     mkdir -p "$PREFIX_PATH"
-    cp -R build/host/* "$PREFIX_PATH"
+    cp -fR build/host/* "$PREFIX_PATH"
     cd "$PREFIX_PATH/bin"
     ln -fs mruby ruby
     ln -fs mirb irb