Remove readarray from `install.sh` (#1198)
## Summary
This isn't available on macOS (see, e.g.,
https://stackoverflow.com/questions/23842261/alternative-to-readarray-because-it-does-not-work-on-mac-os-x),
but this version works both on macOS and Linux.
Closes https://github.com/astral-sh/puffin/issues/1196. (Verified
locally and on CI.)
