Builds: set length on unhealthy builds

[humitos]

Infer the build length by using the first command's start time.

[humitos]

I found we want to use the same logic in the ext-theme as well: readthedocs/ext-theme#644

[ericholscher]

I wonder if we should put -1 or something for build length, since it's a failure mode, and then display something different in the UI?

[humitos]

That's also a good point. Currently, the Build.length=None when the build is terminated. However, the UI is showing detecting that as a special case and showing a dynamic time based on Build.date which is not accurate (I opened an issue about that). Maybe fixing that issue in the template is all we need.