SBCL 2.3.8 didn't build on riscv64, so instead version 2.3.7 is used for
now. It has been merged in master at
4f35ff1275e05be31f5d41464ccf147e9dbfd016.
Let's close this issue and open a new one for a future update of SBCL
(maybe 2.3.9 will fix the build failure).