Fix execute permissions for flatc binary extracted from PAR files (#18738)

[navsud]

Summary:

When binaries are extracted from PAR files via importlib.resources.as_file(),
execute permissions are not preserved, causing PermissionError when trying to
run them.

This change ensures the flatc binary has execute permissions after extraction.

Files fixed:

• executorch/exir/_serialize/_flatbuffer.py: _run_flatc - ensure flatc binary has execute permissions

Reviewed By: abhinaykukkadapu

Differential Revision: D99780508

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/18738

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❌ 3 New Failures, 2 Unrelated Failures

As of commit 95462f4 with merge base 71db7c7 ():

NEW FAILURES - The following jobs have failed:

• pull / test-arm-cortex-m-size-test (zephyr-preset) / linux-job (gh)
  RuntimeError: Command docker exec -t 0da241313335d4951ec6cd45cc4c94a9a0ee4a4fe2aa07f121303e11174ef78b /exec failed with exit code 92
• pull / test-models-linux (mobilebert, portable, linux.2xlarge) / linux-job (gh)
  RuntimeError: Command docker exec -t 06c8739987dba77e4142beb2c2aee1cf9c0b2ab5e56f299f2f86384b6c4e4a65 /exec failed with exit code 1
• pull / unittest-editable / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_mv3_model

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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[meta-codesync]

@navsud has exported this pull request. If you are a Meta employee, you can view the originating Diff in D99780508.