gitlab CI: route stderr/stdout into two different files

The tool prints to stderr for failures, stdout for success. So far we only collected stdout but not stderr. For undefined keysyms we're highly likely to get a whole bunch of failed keymaps, so the log exceeds the gitlab limits.

Let's collect this as files instead, and collect it on failure. We don't really care about the keymaps on success anyway - no-one will look through gigabytes of keymaps in the hope of finding something wrong.