cmd/dist: make.bash -distpack fails

[heschi]

At 265d19e, distpack builds are failing:

distpack: unexpected source archive file: go/src/internal/platform/zosarch.go (generated by dist)

cc @bcmills, since someone mentioned http://go.dev/cl/483695 might be related.

[ianlancetaylor]

I think that CL 490655 will fix this.

[gopherbot]

Change https://go.dev/cl/490655 mentions this issue: cmd/distpack: remove internal/platform/zosarch.go