public-mirror.server (publishConfig.publicMirrorAcr.server) is now required after the change to run syft directly

PR https://github.com/dotnet/docker-tools/pull/1787 adds an unconditional dependency on having a public mirror defined:

https://github.com/dotnet/docker-tools/blob/7841dec2366245a4446a48617e7fbf68b27fa98e/eng/common/templates/jobs/build-images.yml#L107

In the microsoft/go-images pipelines, we define `public-mirror.server` to be `''`. This has seemed reasonable because there's some logic elsewhere that detects/handles it:

https://github.com/dotnet/docker-tools/blob/7841dec2366245a4446a48617e7fbf68b27fa98e/eng/common/templates/steps/init-matrix-build-publish.yml#L26-L28

So, our build fails when trying to upgrade past it: https://dev.azure.com/dnceng/internal/_build/results?buildId=2787270&view=logs&j=fc59f0f2-c1bd-58ae-b870-833d1e8a924c&t=42b16712-47b7-5d4e-b8e4-5c4616c57a47&l=133

What's the best path forward for us?

(I'm not sure what it takes to maintain a public mirror--I'm not necessarily against it. I'm just not sure if it's necessary for our attempting-to-be-low-maintenance reuse of the .NET Docker infra. 🙂 Dev time is still much costlier than build time in our space.)