[flang] Account for .exe suffix in test/Driver/omp-driver-offload.f90.

I believe this test started failing pre-commit testing after D148038.
For example:
https://buildkite.com/llvm-project/premerge-checks/builds/148654#0187b72b-7360-4c52-be94-93eefcb269f4
https://buildkite.com/llvm-project/premerge-checks/builds/148642#0187b691-15a7-44c4-ae23-c7e97ec67755

On Windows the .exe suffix is used for clang-offload-packager tool:
"c:\\program files\\llvm\\bin\\clang-offload-packager.exe" ...

At the same time, I believe I have seen this test sporadically fail before,
meaning that the .exe suffix is not added consistently. I am not sure
why this might be happening. I decided just to allow the suffix.