With the new link to the 2020 toolchain, "riscv-pk" seems to be missing and the following command fails:
time spike /opt/riscv/riscv64-unknown-elf/bin/pk mnist_riscv
In my case there is no pk executable at that location ( i checked other locations within opt/riscv/ but no luck )
I tried to separately install riscv-pk but the installation fails on some unsupported CSRs, so I checkout out to the last commit from 2020. Then it works.