Add kube version argument

[oliver-equinix]

trivy has a helm-kube-version which by default is currently 1.20. When scanning charts if the chart has a minimum chart version higher than the helm-kube-version the scan returns an error.

trivy supports setting this version with --helm-kube-version. This pull request adds the ability to set --helm-kube-version in the plugin for example:

- label: ":trivy: scan"
  command: "echo '--- Running the Trivy security scanner'"
  plugins:
    - equinixmetail-buildkite/trivy#v1.20.0:
        kube-version: "1.21"

[prezha]

looks like trivy accepts helm-kube-version flag regardless of which subcommand or scanner is used, so we should be ok