Starting go 1.16, go build will not update go.sum automatically and requires that users explicitly execute go mod download.

Since jsii/superchain now includes go 1.16 instead of 1.15, pacmak is now failing to create go bindings with the following error:

go: github.com/aws/jsii-runtime-go@v1.21.0: missing go.sum entry; to add it:
 	go mod download github.com/aws/jsii-runtime-go

To address, we need to run go mod download before we run go build.