Replace %I [01, 12] with %H [0, 23]

As indicated here: https://github.com/buildbot/buildbot/commit/b0bcb9edd1a63b6be5b9119992ff41cbdad0989d#commitcomment-8287511

%I is [01,12].

Either we want to replace it with %H [00, 23], or we want to add %p [AM/PM].

This is a fix for that.