Deprecate spec.builder in the Build API

[adambkaplan]

The Build API includes a spec.builder field, which lets users specify the "builder image" for a particular build strategy.
This field was initially added to support build strategies like source-to-image, which can feed in a base image to use for the build.

There are few build strategies that operate like this - Cloud Native Buildpacks might be the closest to using this approach, however our current strategies for buildpacks rely on separate strategies per build pack provider. Moreover, with the Parameters feature we can do away with this specialized API object and use more general-purpose parameters to inject the builder image. For example, the source-to-image build strategies can declare a BUILDER_IMAGE parameter that must be provided in a Build or BuildRun.

/kind feature

/kind cleanup

[adambkaplan]

Note that this issue should require a SHIP

[adambkaplan]

@SaschaSchwarze0 noted that our current API also allows us to specify pull secrets for the builder image. For this to be fully deprecated, we need an alternate mechanism to provide pull secrets for the builder image. Using Build strategy volumes in combination with Build parameters is one potential solution.