Allow re-using previously built packages if no changes were detected

[pzakha]

The build should be able to re-use previously built packages if no changes were detected.

This would speed-up ab-pre-push runs and potentially unblock reduce friction when flakey packages do not need to be rebuilt as often.

There are some limitations on when a package can be re-used and it depends on what triggers are used to detect there were no changes. The latest build from ops.jenkins would be used as a reference and source for previously built packages.

A package would have to be rebuilt if:

1. linux-pkg's repository hash has changed
2. the package's repository hash has changed
3. The package depends on kernel and the kernel version has changed