Explicitly mention that BuildEnv L2 build platform MUST verify the SLSA Provenance OR its VSA.

[marcelamelara]

Prior to the instantiation of a new build environment, the SLSA Provenance for the selected build image MUST be automatically verified.

If it must be, do we have to say what about the image is verified?
Just the VSA's "build level 2" claim about the build image?

Originally posted by @zachariahcox in #1115 (comment)

[paveliak]

@marcelamelara Curious, how does this requirement go along with the claim that we do not assume Control Plane to be trusted at L2? If we demand provenance verification from the Control Plane how can we trust the verification result? And once we add Control Plane into TCB then request for having Control Plane provenance becomes reasonable...