fix: Test_runImportCmd()

[jinsan-line]

Description

• fix Test_runImportCmd().
• It's success only first time but failed after second execution.
• W/ CI environment, it's not issue because it's executed always w/ fresh docker container.

The reason why it's failed.

• If we set FlagKeyringBackend, the keyring is overwritten by cmd.
• If keyring is overwritten, the keyring uses home/keyring... dir but not a temp dir.
• It fails to delete a test key because the keyring for test is overwritten.
• So, after second execution, it fails due there is already same key.

Reference

https://github.com/line/lbm-sdk/blob/c508498905f249c39ce36ca3c45949546dcafb63/client/cmd.go#L119-L130

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