lang: ast: Ensure a list doesn't sneak through type interpolation

If we had a single list wrapped in an interpolated string, it could
sneak through type unification, which is not correct. Wrapping a
variable by interpolation in a string, must force it to be a string.

lang/ast/structs.go

@@ -5408,6 +5408,10 @@ func (obj *ExprStr) Interpolate() (interfaces.Expr, error) {
 		}, nil
 	}
 	// we got something, overwrite the existing static str
+	// ensure str, to avoid a pass-through list in a simple interpolation
+	if err := result.SetType(types.TypeStr); err != nil {
+		return nil, errwrap.Wrapf(err, "interpolated string expected a different type")
+	}
 	return result, nil // replacement
 }