nogo: plumb SDK Go version into go/types (#4564)

Forward the selected SDK Go version from compilepkg into nogo and
initialize the corresponding go/types version plumbing during type
checking. This keeps nogo aligned with the active toolchain version
without keying the action on unrelated gcflags.

Keep gc_goopts -lang overrides unsupported for now. RunNogo still
forwards only the SDK version, and compilepkg now documents that -lang
parsing may be added later once nogo has proper gc flag parsing.

Add focused nogo coverage that checks both the RunNogo action