clientagentcmd: use "localhost" to call client agent (#4816)

The client agent serves on "localhost:9300" by default. It appears
on some systems this resolves to ::1:9300 and on some systems
127.0.0.1:9300. We should use the same address on the client
as the server to avoid any incompatibilities between systems.

internal/cmd/commands/clientagentcmd/clientagentcmd.go

@@ -68,5 +68,5 @@ func (c *ClientAgentCommand) Run(args []string) int {
 
 // clientAgentUrl constructs the full URL for a client agent request given a port and path.
 func clientAgentUrl(port uint, path string) string {
-	return fmt.Sprintf("http://127.0.0.1:%d/%s", port, path)
+	return fmt.Sprintf("http://localhost:%d/%s", port, path)
 }