Create temp file in target directory to prevent cross-device link error

[AsherJingkongChen]

Fixes #1827

Creates temp file in the same directory as the target to prevent EXDEV errors
when temp and target are on different filesystems (e.g., checkpoints to NFS).

Changes

• mkstemp uses dir=os.path.dirname(self.path) with filename prefix
• Removed PermissionError workaround since EXDEV won't occur
• chmod catches PermissionError for filesystems without permission support
• Test verifies temp file location directly

Context

Similar approach was in #1829 initially. As noted there, this only affects local.py.

[martindurant]

Would you say there are times to prefer /tmp over the target directory, for instance for NFS mounts that are slow and don't like many writes?

[martindurant]

If we are not using a system temporary location, we could just use normal open() and avoid the chmod call further down. The only thing mkstemp is doing for us here is generating the filename with some uuid.

[AsherJingkongChen]

keeping mkstemp is fine since it still works and avoids breaking umask cache tests

[AsherJingkongChen]

Would you say there are times to prefer /tmp over the target directory, for instance for NFS mounts that are slow and don't like many writes?

@martindurant Both approaches write filesize to NFS. This PR just skips the extra copy step. Can't think of a case where /tmp would be better.