Protect .cargo-ok on case-insensitive file systems

[kornelski]

Tar decompression is trying to forbid .cargo-ok from being expanded from the archive, but it's performing an exact string check, which is effective only on case-sensitive file systems. On macOS and Windows a .CARGO-OK (or some other combination of upper case letters) can create a file that has the same path as .cargo-ok.

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[weihanglo]

I believe this line already protects them from overwrites, though your patch seems also reasonable. Could you add a test to prevent future regression?

[epage]

When adding the test, please make it its own commit first and having passing, showing the bad behavior. Then in the commit with your fix, it will be updated to show the change in behavior.