Properly kill the buck2 daemon

[dbort]

Fix the buck2 kill command. Because of scoping issues, in some cases we only ran " kill" because the local value of $BUCK2 was empty.

This should help avoid failures like

  Error validating working directory
  Caused by:
      0: Failed to stat `/home/ubuntu/cmodi/executorch/buck-out/v2`
      1: ENOENT: No such file or directory

which are typically fixed by running buck2 kill.

Add "COMMAND_ECHO" to the kill command to show what we're running.

Also, make the function consistently use executorch_root as the working directory. We should always run buck2 from the ET repo.

Test Plan:

rm -rf buck-out
(rm -rf cmake-out \
    && mkdir cmake-out \
    && cd cmake-out \
    && cmake ..)

Output showed that the local variable works:

-- Resolved buck2 as /Users/***/local/executorch/cmake-out/buck2-bin/buck2-99773fe6f7963a72ae5f7b737c02836e.
-- Killing buck2 daemon
'/Users/***/local/executorch/cmake-out/buck2-bin/buck2-99773fe6f7963a72ae5f7b737c02836e kill'

And that the parent variable still works:

-- ******** Summary ********
...
--   BUCK2                         : /Users/***/local/executorch/cmake-out/buck2-bin/buck2-99773fe6f7963a72ae5f7b737c02836e

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/5373

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✅ You can merge normally! (1 Unrelated Failure)

As of commit f278a60 with merge base a91eb8a ():

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

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

• trunk / test-coreml-delegate / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1

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[facebook-github-bot]

@dbort has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[dbort]

Didn't fix it. https://github.com/pytorch/executorch/actions/runs/10858643573/job/30136900450?pr=5373#step:9:651

  -- Resolved buck2 as buck2.
  'mkdir -p '/Users/ec2-user/runner/_work/executorch/executorch/pytorch/executorch/buck-out/v2''
  -- Killing buck2 daemon
  'buck2 kill'

...

  Caused by:
      0: Failed to stat `/Users/ec2-user/runner/_work/executorch/executorch/pytorch/executorch/buck-out/v2`
      1: ENOENT: No such file or directory

[dbort]

Reducing the scope of this PR to fix the buck2 kill part since that's still helpful.

[facebook-github-bot]

@dbort has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[dbort]

@pytorchbot cherry-pick --onto release/0.4 -c critical