We should regularly run against bitcoind master, to detect if a change to
bitcoind is affecting us.
This will let us detect changes that may break lightning before bitcoind
releases them, giving us a chance to rectify it.
We really don't want to run this workflow for every pull request or every
merge to master, we instead run it twice per week, which should give
us enough time to detect dangerous changes.
The default is 360 minutes which is very long.
Our builds are usually around 10 minutes, so 20 minutes is a good value
to prevent overbilling builds that create infinite loops.
These tests have been flaky because external APIs tend to be down or throttle
our calls. and we don't want our test suite to fail because of that.
In practice it's enough if developers run these tests locally once in a while.
