cmd/go: print newline after GOOS/GOARCH error

The newline was dropped during the refactor in CL 194617.

Fixes #35984

Change-Id: I7e0d7aa2d7a4d1f44898921f8bb40401620d78b2
Reviewed-on: https://go-review.googlesource.com/c/go/+/209965
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/internal/work/action.go

@@ -291,7 +291,7 @@ func (b *Builder) Init() {
 	}
 
 	if err := CheckGOOSARCHPair(cfg.Goos, cfg.Goarch); err != nil {
-		fmt.Fprintf(os.Stderr, "cmd/go: %v", err)
+		fmt.Fprintf(os.Stderr, "cmd/go: %v\n", err)
 		base.SetExitStatus(2)
 		base.Exit()
 	}