fix: default TEMPDIR to system temp so S3 works with read-only CWD

[Shriprasad-P]

Fixes

Fixes #854

Summary

Default TEMPDIR to tempfile.gettempdir() instead of ., so the S3 client no longer requires the current working directory to be writable.

Test plan

• Added unit test that instantiates S3 from a read-only CWD
• Existing S3 unit tests still pass

[greptile-apps]

Greptile Summary

This PR changes the default Metaflow temporary directory from the current working directory to Python’s system temporary directory, allowing S3 initialization from a read-only working directory.

• Preserves explicit METAFLOW_TEMPDIR and tmproot overrides.
• Updates S3 documentation and remote-runtime comments to describe the new default.
• Tests default and configured behavior in isolated subprocesses, avoiding import-time state leakage.

Confidence Score: 5/5

The PR appears safe to merge; no actionable new issue or outstanding previous finding remains.

The current implementation preserves configured overrides, uses the system temporary directory by default, and isolates import-time configuration checks in subprocesses. The earlier state-leak finding was manually resolved without an explanatory reply, and the current subprocess implementation removes the shared module-reload behavior that caused it.

Important Files Changed

Filename	Overview
metaflow/metaflow_config.py	Defaults TEMPDIR to tempfile.gettempdir() while preserving configuration overrides.
metaflow/plugins/datatools/s3/s3.py	Updates the S3 API documentation to accurately describe temporary-directory precedence.
metaflow/plugins/aws/batch/batch_decorator.py	Updates the Batch runtime comment to reflect the system temporary-directory default.
metaflow/plugins/kubernetes/kubernetes_decorator.py	Updates the Kubernetes runtime comment to reflect the system temporary-directory default.
test/unit/test_s3_readonly_cwd.py	Adds subprocess-isolated coverage for read-only working directories and explicit temp-directory overrides.

_{Reviews (4): Last reviewed commit: "Isolate S3 tempdir tests in subprocesses" | Re-trigger Greptile}