Use the install method for flatc

[jathu]

Summary

I came across this "cleaner" approach when setting up flatcc (#10855). So, let's do it for flatc too.

Test plan

CI

cc @larryliu0820

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/10859

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❗ 1 Active SEVs

There are 1 currently active SEVs. If your PR is affected, please view them below:

• [PREEMPTIVE] Removal of ephemeral variants on scale-config.yml

❌ 1 New Failure

As of commit 2192226 with merge base 518324f ():

NEW FAILURE - The following job has failed:

• trunk / test-arm-backend (test_pytest_ops_ethosu_fvp) / linux-job (gh)
  RuntimeError: Command docker exec -t e46170a3678b50735da3e1f96af4884c71a93a73cccea0ccbd6ef503ecaf22f1 /exec failed with exit code 1

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