Sorry for reading too fast.
This should work for a packages.txt file:
chromium
chromium-driver
I just tested an app and didn’t have any problems installing chromium. Have you rebooted the app? Deleted it and redeployed it? If you remove the pins on your other dependencies, is there any difference? Community Cloud is using Debian Linux Bullseye, so that might be forcing some things to be older versions than in your development environment.