[RISCV] Pass -DPython3_EXECUTABLE=/usr/bin/python3 for rva20-2stage builder

This avoids issues where the python3 executable is at a different path
in the host vs the guest environment (i.e. under qemu-system used for
lit execution).

Author: asb

buildbot/osuosl/master/config/builders.py

@@ -3270,6 +3270,7 @@
                 extra_stage2_cmake_args=[
                     util.Interpolate("-DLLVM_NATIVE_TOOL_DIR=%(prop:builddir)s/stage1.install/bin"),
                     "-DLLVM_BUILD_TESTS=True",
+                    "-DPython3_EXECUTABLE=/usr/bin/python3",
                     util.Interpolate("-DLLVM_EXTERNAL_LIT=%(prop:builddir)s/llvm-zorg/buildbot/riscv-rise/lit-on-qemu")],
                 stage2_toolchain_options=[
                     "set(CMAKE_SYSTEM_NAME Linux)",