Add _ENABLE_VGF toggle to test targets

[3l1]

Summary:
Add a simple boolean toggle (_ENABLE_VGF) to the ARM test targets.bzl
so VGF backend dependencies (env vars, preload_deps) can be quickly
disabled if needed without removing code.

Reviewed By: Ninja91

Differential Revision: D99479233

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/18720

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❌ 2 New Failures, 63 Pending

As of commit b859193 with merge base 19bbeac ():

NEW FAILURES - The following jobs have failed:

• Cadence Build & Test / cpu-test / test-aot / test-aot (gh)
  backends/cadence/aot/tests/test_replace_ops_passes.py::TestReplaceOpsPasses::test_replace_conv2d_with_linear
• Cadence Build & Test / cpu-test / test-ops / test-ops (gh)
  examples/cadence/operators/test_g3_ops.py::ATenOpTestCases::test_g3_neg_out_4

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[meta-codesync]

@3l1 has exported this pull request. If you are a Meta employee, you can view the originating Diff in D99479233.

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