cmd/go: `go env GOMOD` tries to download newer toolchain

If `go.mod` says

```
module example.com

go 1.22
```

Then running `go env GOMOD` with 1.21 prints:

```
$ go env GOMOD
go: downloading go1.22 (linux/amd64)
go: download go1.22 for linux/amd64: toolchain not available
```

This is a bit silly, as `go env` needs to determine `GOMOD` in order to decide to upgrade in the first place.

cc @bcmills 