Fix typo ${tolchain} in build_executor_runner.sh

[armwaheed]

executorch/backends/arm/scripts/build_executor_runner.sh

Line 78 in f332196

echo "Error: Invalid toolchain selection, provided: ${tolchain}"

Please fix typo ${tolchain} and replace it with ${toolchain}

This is related to issue #12858

Thanks!

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218

[digantdesai]

@armwaheed #12888 FYI