build_package_mruby() doesn't force cp command

When running `rbenv install -f 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.

Here's the offending line:
https://github.com/sstephenson/ruby-build/blob/master/bin/ruby-build#L441

Adding `-f` to the `cp` command fixed the issue for me.