should extract build id in another way when build status is unknown

[gcb-repos-robot]

In chat notifier, we extract the build id from the first 8 chars in the build id Here. But build id is empty when the build has status unknown and would throw this error:

http: panic serving 169.254.1.1:35855: runtime error: slice bounds out of range [:8] with length 0 goroutine 47

We should manually set the build id when the build status is unknown.

[vicpadilla]

cc @monicacinom