fix security vulnerability in extracting chart tarballs

[prydonius]

See https://github.com/kubernetes/kubernetes/issues/61297, the solution is to use https://github.com/cyphar/filepath-securejoin to ensure the path is within context.

Issue is in https://github.com/kubernetes-helm/monocular/blob/8c4657befc836badf3a717c4cb215768b0671e28/src/api/data/cache/charthelper/chart_package_helper.go#L205

See fix in Helm: https://github.com/kubernetes/helm/pull/3898