Add a standalone SimpleNet example build to the CI

[jwallwork23]

This PR adds an additional CI step to build and run one of the examples in a "standalone" fashion. This checks that we can build it with CMake separately from FTorch without needing to set LD_LIBRARY_PATH etc.

Checklist

• Ubuntu
• Windows
• Mac?

[jwallwork23]

Huh, surprised that this fails as the following script works fine for me locally

#!/bin/bash

FTORCH_BUILD_DIR="$(pwd)/build"
cd examples/1_SimpleNet
export BUILD_DIR="$(pwd)/build"
rm -rf "${BUILD_DIR}"
mkdir "${BUILD_DIR}"
cd "${BUILD_DIR}"
cmake .. \
  -DPython_EXECUTABLE="$(which python)" \
  -DCMAKE_BUILD_TYPE=Release \
  -DCMAKE_PREFIX_PATH="${FTORCH_BUILD_DIR}" \
  -DCMAKE_BUILD_TESTS=TRUE
cmake --build .
ctest -V

[jatkinson1000]

Yep, pretty sure this matches what I was getting.
Interesting.