It appears that this problem was fixed in ROCm 5.2 [1].
[1]: https://github.com/ROCm-Developer-Tools/HIP/commit/d221fb6ebbe24d66855af8160687aa01b0112dec
My mistake. The change in ROCm 5.2 fixes the case when rocm_agent_enumerator reports that the system has no GPUs, but not the case when rocm_agent_enumerator returns no output due to an error. The latter is what's occurring on the autobuilders, so it seems that you will need to specify the OFFLOAD_ARCH_STR even once the package gets updated to ROCm 5.2.
Sincerely,
Cory Bloor