[camera_windows] Fall back to temp dir for capture path.

[llfbandit]

Fixes flutter/flutter#192302

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No test added since there's actually no test on this path.

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[gemini-code-assist]

Code Review

This pull request updates the Windows camera plugin to fall back to the system temp directory for saving captured pictures and videos when the standard Pictures or Videos known folders cannot be resolved. The feedback suggests updating a comment to clarify that this fallback will not be triggered if SHGetKnownFolderPath succeeds but write access is blocked, such as by Controlled Folder Access.