Use reference to the response rather than copying it

[sosthene-nitrokey]

This should reduce stack usage, see Nitrokey/nitrokey-3-firmware#312

[sosthene-nitrokey]

Looking at the results of -Z emit-stack-sizes, this reduces this function's stack usage from 22920 bytes of stack used to just 56!
I'm still confused as to why it was so high in the first place. Only one message was ever stored on the stack, so it should be closer to 7KiB. I see similar weirdness in other functions, where the stack usage is multiple times that of the main values stored in it.