Minor update, mariadb is also failing to build on my desktop now. So it looks like when it managed to complete a few weeks ago was lucky timing and not some configuration difference between my laptop and desktop.
I've also attached the log from my desktop, which is much the same as the ones from my laptop. The only failed test was "main.mysqldump", which failed in a lot of ways, but I am not familiar enough with the program or tests to make heads or tails of the errors or have any idea how to fix.

I also checked "guix gc --referrers" on mariadb, and it looks like qtbase is pulling it in.
Sadly, even after installing portgresql and mysql, guix kept trying to build mariadb, so it looks like qtbase is specifically pulling in mariadb and not just a generic database engine, so I still can't install stuff like SDL, ffmpeg, calibre, and miscellaneous other stuff, until mariadb decides to build successfully.