Fix directory and files permission on `unix` platform

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This PR want to:

• [x] set keystore directory's permission to drwx------
• [x] set files under keystore's permission to -rw-------
• [x] set exported file by ckb-cli account export's permission to -r--------

Request for review: @zhangsoledad @doitian

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Please trigger the github CI action.

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This failed test is unrelated to this PR. It may be fixed by this

I just fixed cargo clippy, please re-trigget the CI action.

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need rebase...