Same issue here specifically at the end of the composer install step, even re-running a previously passing build causes the error so clearly nothing we’ve changed ourselves.
For those of you who raised a support ticket, you should have had an explanation already.
For the rest of you: there was a bug related to how the PHP binary in CLI mode handles closing the output stream before the process ends, which caused us to inadvertently abort the process early.
We have since fixed this bug and added a regression test for ths unusual PHP behaviour which should avoid any future reocurrences.