"git ubuntu build" races container readiness

In 7e79017 (actually a feature branch based off that), I'm trying to use "git ubuntu build" to test that quiltification has not regressed.

I found that the call to "apt update" (which I changed to "apt-get update" due to bug 1734366) was returning immediately, but not in my manual testing.

This appears to be a race. Adding a "import time; time.sleep(20)" before the apt-get update retry loop works around the problem.

The failure is that the subsequent "apt-get install" (also changed from plain "apt install") cannot find packages because the index update apparently never took place, and this leads to a failure of the build.