Fix repo reference in az Dockerfile

[mthalman]

The build for the changes from #1068 is failing because of the --registry-override option that is used. (This didn't fail in the PR build because that option is not used for the build.) The Dockerfile.az.linux file had hardcoded the image names it referenced in its FROM instructions. In conjunction with the --registry-override option, it caused Image Builder to rewrite the file to apply the override. But it generated a FROM instruction like this: FROM dotnetdocker.azurecr.io/dotnet-buildtools/image-builder:linux-$ARCHAS az-installer. Note the lack of a space between $ARCH and AS. This isn't valid syntax so the build failed.

We shouldn't be having the Dockerfile be rewritten with the registry option. We want to keep the exact version that exists in the repo. To fix this, we can replace the repo reference in the Dockerfile with a reference to an ARG that is dynamically set within the manifest file. This follows the same convention that is used in the manifest of https://github.com/dotnet/dotnet-docker.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.