Specify Python3 executable path in CI (#11696)

diff --git a/build_tools/cmake/build_riscv.sh b/build_tools/cmake/build_riscv.sh
index 10d20ff..7d5ff6c 100755
--- a/build_tools/cmake/build_riscv.sh
+++ b/build_tools/cmake/build_riscv.sh
@@ -38,13 +38,15 @@
 args=(
   "-G" "Ninja"
   "-B" "${BUILD_DIR}"
-  -DCMAKE_TOOLCHAIN_FILE="${TOOLCHAIN_FILE}"
-  -DIREE_HOST_BINARY_ROOT="${IREE_HOST_BINARY_ROOT}"
-  -DRISCV_CPU="${RISCV_PLATFORM_ARCH}"
-  -DRISCV_COMPILER_FLAGS="${RISCV_COMPILER_FLAGS}"
-  -DIREE_BUILD_COMPILER=OFF
+  "-DPython3_EXECUTABLE=${IREE_PYTHON3_EXECUTABLE}"
+  "-DPYTHON_EXECUTABLE=${IREE_PYTHON3_EXECUTABLE}"
+  "-DCMAKE_TOOLCHAIN_FILE=${TOOLCHAIN_FILE}"
+  "-DIREE_HOST_BINARY_ROOT=${IREE_HOST_BINARY_ROOT}"
+  "-DRISCV_CPU=${RISCV_PLATFORM_ARCH}"
+  "-DRISCV_COMPILER_FLAGS=${RISCV_COMPILER_FLAGS}"
+  "-DIREE_BUILD_COMPILER=OFF"
   # CPU info doesn't work on RISCV
-  -DIREE_ENABLE_CPUINFO=OFF
+  "-DIREE_ENABLE_CPUINFO=OFF"
 )
 
 if [[ "${RISCV_PLATFORM}" == "linux" ]]; then