always set buildkitd-flags if opt-in

[crazy-max]

follow-up #357 (comment)

We should always pass buildkitd-flags if user opt-in for it and let the driver handle it. We just need to make sure the driver supports buildkitd-flags if the input is not set to pass defaults.

[tonistiigi]

Can we make sure that the docs mention that if user sets buildkitd-flags then the default is reset, and user might want to make sure set default again if they want that behavior.

[crazy-max]

@tonistiigi Done

[twelho]

From what I can tell, this prevents clearing of buildkitd-flags if the user does not want the default insecure entitlement flags. Setting something like

- uses: docker/setup-buildx-action@v3
  with:
    buildkitd-flags: ""

or any other falsy value will get disregarded, and the insecure entitlement flags will always get applied, which is unwanted in some situations.