cmd/go: go version stopped getting added to go.mod

[rsc]

I ran some go build commands just now in a new module and there is not a go version in my go.mod. There should be.

/cc @bcmills @jayconrod

[bcmills]

That was an intentional change in behavior for #30790.

We now add the go directive only in go mod init and go mod tidy

[bcmills]

Per discussion with Russ, we should go with the alternative described in #30790 (comment): we should always write the go version if it isn't present.

[gopherbot]

Change https://golang.org/cl/176925 mentions this issue: cmd/go: always add 'go' directive to the go.mod file if missing