Make environment check dynamic

[gramalingam]

Change the check on environment variable to be at runtime. This allows benchmarking code to set the environment variable as needed before export.

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
14040	1	14039	1610

View the top 1 failed tests by shortest run time

onnxscript.backend.onnx_export_test.TestOnnxBackEnd test_export2python_produces_correct_onnx_script_model_0012_test_affine_grid_2d

Stack Traces | 0.004s run time

onnxscript\backend\onnx_export_test.py:137: in extract_functions
    mod = importlib.import_module(import_name)
C:\hostedtoolcache\windows\Python\3.11.9\x64\Lib\importlib\__init__.py:126: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
E   ModuleNotFoundError: No module named 'tests.onnx_backend_test_code.test_affine_grid_2d'

The above exception was the direct cause of the following exception:
.nox\test\Lib\site-packages\parameterized\parameterized.py:620: in standalone_func
    return func(*(a + p.args), **p.kwargs, **kw)
onnxscript\backend\onnx_export_test.py:271: in test_export2python_produces_correct_onnx_script_model
    functions = extract_functions(backend_test.name, code, self.test_folder)
onnxscript\backend\onnx_export_test.py:139: in extract_functions
    raise AssertionError(
E   AssertionError: Unable to import 'tests.onnx_backend_test_code.test_affine_grid_2d' (e=No module named 'tests.onnx_backend_test_code.test_affine_grid_2d') (file: 'D:\\a\\onnxscript\\onnxscript\\tests\\onnx_backend_test_code\\test_affine_grid_2d.py', absolute path: 'D:\\a\\onnxscript\\onnxscript\\tests\\onnx_backend_test_code\\test_affine_grid_2d.py', current folder: D:\a\onnxscript\onnxscript
E   ---- CONTENT --
E   import numpy
E   from onnx import TensorProto
E   from onnx.helper import make_tensor
E   from onnxscript import script, external_tensor
E   from onnxscript.values import Opset
E   from onnxscript.onnx_types import FLOAT, INT64
E   from onnxscript.onnx_opset import opset20
E   
E   @script()
E   def bck_test_affine_grid_2d(theta: FLOAT[2,2,3], size: INT64[4]) -> (FLOAT[2,5,6,2]):
E       grid = opset20.AffineGrid(theta, size, align_corners=0)
E       return grid

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