Slackbot notification builder ignores the time out of the build is monitoring

Hi there : )

When triggering the secondary build to monitor the state of the main build, the timeout of the main build is ignored.

So even if in my `cloudbuild.yaml` I set up a build time out of 900s, if my build happens to go over 600s no notification will be sent because the monitoring build will time out after 600s.

I've quickly hacked this in `trigger.go` hardcoding my own time out when creating the trigger:

```
Tags: []string{"slackbot"},
Timeout: "900s", // <--- I added this
```

Ideally Slackbot should pull the timeout information from the Cloud Build API in order to use the same or maybe slightly higher. But I haven't checked that this is possible and my Go knowledge is non-existent!

Thanks,

Juan