The ruby post-install hook was not run for slim images

[eregon]

As we can see https://github.com/graalvm/container/blob/952ceb4186763bd79c340b636adf89ca7267d2a5/truffleruby/Dockerfile.ol8-slim doesn't run it, unlike https://github.com/graalvm/container/blob/952ceb4186763bd79c340b636adf89ca7267d2a5/truffleruby/Dockerfile.ol8#L29

This then cause errors whenever the openssl gem is used, including just loading the gem.

Probably the easiest fix is to build the slim images from the non-slim images, and just don't copy the lib/llvm-toolchain. Alternatively, installing the necessary packages to run the post-install hook, run it and uninstalling these packages might be another way.