Consider setting RUBY_BUILD_CACHE_PATH?

Hi there! I recently ran into some trouble installing ruby on a fedora vm, and in the process of resolving the build errors I ran `asdf install` several dozen times.

Every time I attempted to install ruby with asdf, it downloaded a new collection of artifacts into a timestamped folder. This slowed down my debugging process, and a handful of times in the process I ran into "no space on device" errors and found that the disk backing /tmp was full. Manually clearing out the cache is easy enough, but seems like it should be unnecessary.

I posted on [ruby-build](https://github.com/rbenv/ruby-build/discussions/2463) about the process and the maintainer there mentioned RUBY_BUILD_CACHE_PATH. A [quick search here](https://github.com/search?q=repo%3Aasdf-vm%2Fasdf-ruby%20RUBY_BUILD_CACHE_PATH&type=code) didn't turn anything up -- perhaps this would be an easy way to decrease the suffering of someone who is already fighting with the computer to get ruby installed?