Handle missing accelerate executable in env command

[tandede]

Summary

• replace the platform-specific which/where subprocess with shutil.which
• preserve the existing Not found diagnostic when the console script is not on PATH
• add regression coverage for invoking the environment command without an accelerate executable on PATH

Why

accelerate env initializes the reported executable location to Not found, but then unconditionally runs an external lookup command. When Accelerate is invoked through an explicit path without activating its environment, that lookup exits non-zero and subprocess.check_output() raises CalledProcessError. As a result, the diagnostic command crashes instead of reporting the already-intended fallback.

Using shutil.which() provides the same lookup on POSIX and Windows while allowing a missing executable to be handled normally.

Validation

• PATH=/usr/bin:/bin accelerate env (reports `accelerate` bash location: Not found)
• pytest -q tests/test_cli.py -k 'EnvCommandTester or test_hyphen or test_underscore' (3 passed)
• pytest -q tests/test_memory_utils.py (7 passed)
• pytest -q tests/test_cli.py -k 'not test_invalid_model_name_transformers' (38 passed, 1 skipped, 5 subtests passed)
• make style
• make quality