chore: set User-Agent on all outgoing HTTP requests (#10867)

It's only polite, and it enables better troubleshooting in situations
like weird-ass requests to the upgrades download service.

---------

Signed-off-by: Jakob Borg <jakob@kastelo.net>

lib/ur/failurereporting.go

@@ -214,6 +214,7 @@ func sendFailureReports(ctx context.Context, reports []contract.FailureReport, u
 		return
 	}
 	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("User-Agent", build.UserAgent())
 
 	resp, err := client.Do(req)
 	if err != nil {