What causes CircleCI to decide to install Ruby?


I have one Rails project that early in the build executes a shell script (ending in .sh). When it gets to that step, before executing the script, it takes a break to install ruby from RVM. What triggered that install?

In another project, I have several shell scripts, no Gemfile or .ruby-version, and it does the same thing, installing ruby before it runs the scripts. What caused Circle to decide that it needed to install ruby before it could run my script? Is there a way to disable it?