Use dictionary for storing build logs instead of writing files

This MR drops writing out multiple *build.log files containing logs collected by lb-wrapcmd in favour of storing them in dictionary with the following format:

{"subdir": [("log_id", "log_content"), ...], ...}

to minimise rewrite of build_log.py code ported from LbNightlyTools.

It's built on top of !53 (merged). The effective diff is https://gitlab.cern.ch/lhcb-core/nightly-builds/lb-nightly-functions/-/compare/maszyman-async-socket-4...maszyman-logs-no-files.

I marked it as a draft if we decide to go for !50 (closed) (or another solution) so that I'll adapt this MR.