Thanks for reporting this, everyone!
We’ve identified this to be a caching issue where some languages rely on $HOME to cache build artifacts, so when this stage executes a script with a different HOME it starts compiling / downloading dependencies again.
We’re still thinking about how to rectify this… Fixing each language’s reliance on $HOME feels very error-prone and hard for us to get right. Our current most promising idea is to change the tests for this stage to work with a fixed HOME value and test things like cd ~/random_dir.