Tests: fix CMake empty `ALPAKA_CUDA_ARCH`
Tests: fix CMake empty `ALPAKA_CUDA_ARCH`

In `compileExec.sh` cmake variables where set during the execution of `cmake` e.g. `-DALPAKA_CUDA_ARCH="${ALPAKA_CUDA_ARCH}"`.
In the case where the environment variable `ALPAKA_CUDA_ARCH` is not defined the cuda architecture for
cmake will be set to empty. This overwrites our define for the minimum CUDA architecture which we derivein CMake out of the used CUDA Toolkit version. This results into an crash if we use clang as compiler.

Error:
```
clang-4.0: error: cannot find libdevice for sm_20. Provide path to different CUDA installation via --cuda-path, or pass -nocudalib to build without linking with libdevice.
```

BenjaminW3/alpaka,BenjaminW3/alpaka