As suggested by Ludovic on IRC, after this patch, build log files (from
/var/log/guix/drvs) will be displayed in a 'guix-build-log-mode'.

I'm not sure about regexp though: are these files always "bz2" and
".../XX/XXXXX-30-more-characters-XXXXX/..."?

I didn't put "/var/log" in the beginning of the regexp to match, for example, /mnt/guix-from-sdc/var/log/guix/... files or alike.