Fix kubectl-delivery security issues

[ryantd]

Fix gosec G304 G307 security issues

Ref
G304: File path provided as taint input
G307: Unsafe defer call of a method returning an error for defer file.Close() ?

[terrytangyuan]

Thanks!

/lgtm
/approve

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[terrytangyuan]

@ryantd Would you like to add the gosec check to part of the CI?

[ryantd]

Sure, Terry.
Will set up asap after getting familiar with the process of our CI