I think I found the source of the problem. It's not cd'ing into the right directory, so the cmd/* glob isn't expanding. Instead, it's resolving to a literal cmd/*.
+ cd /builddir/build/BUILD + cd mqttcli-0.1.0 + for cmd in cmd/*
If I'm reading the commands executed during %prep correctly, the correct path should be /builddir/build/BUILD/mqttcli-0.1.0/mqttcli-0.1.0. I'll keep studying these go macros to see if it's something I need to tweak to support the git.sr.ht forge.