go/types, types2: use "assignment mismatch: x variables but y values" error message

go/types, types2: use "assignment mismatch: x variables but y values" error message

This matches current compiler behavior.

For #55326.

Change-Id: I7197cf4ce21e614291a1a2e1048dd78d0a232b64
Reviewed-on: https://go-review.googlesource.com/c/go/+/436175
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@google.com>