cmd/go: pass the `-x` output as a `Context` key instead of hard-coding `os.Stderr`

[bcmills]

When we run go build -x, we send a bunch of extra logging to os.Stderr. That's fine in production code, but it makes unit-testing unpleasant: if tests run in parallel with cfg.BuildX set to true, the logging from the tests ends up all jumbled together.

We're already threading through a context.Context argument to many functions for tracing (see cmd/go.maybeStartTrace). We should also use that argument for output streams in order to better isolate tests.

[gopherbot]

Change https://go.dev/cl/452456 mentions this issue: cmd/go: propagate Context arguments through modfetch methods