cmd/go: add -cover flag to report by go version -m

Fixes #67366

Change-Id: Ie39a8ebe7fe2faf59a5f723c69d6d5c8e1a9cd8b
GitHub-Last-Rev: 021134582d
GitHub-Pull-Request: golang/go#67572
Reviewed-on: https://go-review.googlesource.com/c/go/+/587375
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>

src/cmd/go/internal/load/pkg.go

@@ -2349,6 +2349,9 @@ func (p *Package) setBuildInfo(ctx context.Context, autoVCS bool) {
 			appendSetting("-ldflags", ldflags)
 		}
 	}
+	if cfg.BuildCover {
+		appendSetting("-cover", "true")
+	}
 	if cfg.BuildMSan {
 		appendSetting("-msan", "true")
 	}