Fix pip install . -e when buck2 is not installed

[mergennachin]

If the installation goes through regular './install_executorch.sh' path, it sets the -DBUCK2 cmake config, but if you directly call pip install . it
doesn't resolve buck dependency.

Test Plan:

Run

• ./install_requirement.sh and pip install . --no-build-isolation -v
• ./install_requirement.sh and pip install -e . --no-build-isolation -v
• ./install_executorch.sh

cc @larryliu0820 @jathu

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/11842

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✅ You can merge normally! (1 Unrelated Failure)

As of commit 2150e78 with merge base 608a745 ():

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

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

• Apple / build-benchmark-app / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 65

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