Bump toolchains/riscv-tools/riscv-isa-sim from 7812eab to b81b5a0

Bumps [toolchains/riscv-tools/riscv-isa-sim](https://github.com/riscv-software-src/riscv-isa-sim) from `7812eab` to `b81b5a0`.
- [Release notes](https://github.com/riscv-software-src/riscv-isa-sim/releases)
- [Commits](riscv-software-src/riscv-isa-sim@7812eab...b81b5a0)

---
updated-dependencies:
- dependency-name: toolchains/riscv-tools/riscv-isa-sim
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>