internal/vulncheck: emit progress message instead of warning

Now that progress messages are hidden under the verbose flag, we should
not be printing anything with fmt.

Change-Id: I30b247ca1793ea7bcc6e4a68e1f0633d3262d7f1
Reviewed-on: https://go-review.googlesource.com/c/vuln/+/581276
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Run-TryBot: Zvonimir Pavlinovic <zpavlinovic@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Cottrell <iancottrell@google.com>

internal/vulncheck/binary.go

@@ -68,7 +68,10 @@ func binary(ctx context.Context, handler govulncheck.Handler, bin *Bin, cfg *gov
 	}
 
 	if bin.GOOS == "" || bin.GOARCH == "" {
-		fmt.Printf("warning: failed to extract build system specification GOOS: %s GOARCH: %s\n", bin.GOOS, bin.GOARCH)
+		p := &govulncheck.Progress{Message: fmt.Sprintf("warning: failed to extract build system specification GOOS: %s GOARCH: %s\n", bin.GOOS, bin.GOARCH)}
+		if err := handler.Progress(p); err != nil {
+			return nil, err
+		}
 	}
 	affVulns := affectingVulnerabilities(mv, bin.GOOS, bin.GOARCH)
 	if err := emitModuleFindings(handler, affVulns); err != nil {