fix: Protect codersdk.Client SessionToken so it can be updated (#4965)

This feature is used by the coder agent to exchange a new token. By
protecting the SessionToken via mutex we ensure there are no data races
when accessing it.

cli/speedtest_test.go

@@ -22,7 +22,7 @@ func TestSpeedtest(t *testing.T) {
 	}
 	client, workspace, agentToken := setupWorkspaceForAgent(t)
 	agentClient := codersdk.New(client.URL)
-	agentClient.SessionToken = agentToken
+	agentClient.SetSessionToken(agentToken)
 	agentCloser := agent.New(agent.Options{
 		Client: agentClient,
 		Logger: slogtest.Make(t, nil).Named("agent"),