Don't build trust bundle images using make image

[SgtCoDFish]

make image is not as well defined as make trust-package-debian-save or make trust-manager-load, but it seems reasonable to have it available as a test when developing trust-manager.

If make image also applies to the trust package, it requires that users come up with a value for DEBIAN_TRUST_PACKAGE_VERSION which most users won't be able to guess.

Most users should be using the provided commands for loading trust-manager into a kind cluster when developing, which sidesteps this problem entirely.

It feels like the simplest thing here is to have make image be an alias for make trust-manager-save.

[jetstack-bot]

@hawksight: adding LGTM is restricted to approvers and reviewers in OWNERS files.

In response to [this](https://github.com/cert-manager/trust-manager/pull/269#pullrequestreview-1810891493): >/lgtm Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.

[inteon]

/approve /lgtm

[jetstack-bot]

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