fix: remove erroneous lstrip("/") in FileLock path construction

[yangrq1018]

Summary

lstrip("/") on pr.path in MLflowExpManager._get_or_create_exp turns absolute file:// URIs into relative paths, causing mlruns/filelock to be created under a spurious home/<user>/<cwd>/... directory tree relative to the working directory.

• Root cause: In qlib/workflow/expm.py (line 236), urlparse on a local file:///... URI produces netloc="" and path="/home/.../mlruns". The .lstrip("/") strips the leading /, so os.path.join("", "home/.../mlruns", "filelock") yields a relative path instead of the correct absolute path.
• Introduced in: a0cef03 ("update python version update python version #1868") as part of a bulk lint/CI cleanup — not an intentional logic change.
• Original behavior: commit 45ea4ba ("Add file lock for MLflowExpManager Add file lock for MLflowExpManager #619") correctly used pr.path without stripping.

Fix

Remove the .lstrip("/") call. os.path.join("", "/absolute/path", "filelock") already produces the correct absolute path when netloc is empty.

- with FileLock(Path(os.path.join(pr.netloc, pr.path.lstrip("/"), "filelock"))):
+ with FileLock(Path(os.path.join(pr.netloc, pr.path, "filelock"))):

Related note

A similar misuse of lstrip exists in qlib/workflow/recorder.py (lines 321–323), where .lstrip("file:") is used to strip a URI prefix. Since str.lstrip strips characters (not a prefix), this can over-strip in edge cases. The correct approach would be str.removeprefix("file:") (Python 3.9+). This is not addressed in this PR.

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