Co-authored-by: Tom Prince <tom.prince@hocat.ca>

We collect coverage info so we can't very easily run on multiple cores.  So we
mostly can't take advantage of the bigger machine.

With Cachix we can now cache even partial build successes so even if we lose
the whole cache and have to start from scratch, the build should now converge
on success even if it takes a few tries.

hope it does not break the build cachix setup step

Also move this time to a point shortly in my future so it will run and I can
see what happens.

Also, always store the cache because it should at least save us some effort
next time.

This way we get the cache even if the test suite fails

Since I don't want to build another right now, drop it.

Currently builds are failing to upload coverage reports because:

```
==> Uploading
    .url https://codecov.io
    .query service=circleci&package=py2.0.15&job=1057.0&yaml=codecov.yml&token=<secret>&build=1057.0&branch=154.save-and-respond.1&commit=714379bf&slug=PrivateStorageio%2FZKAPAuthorizer
    Pinging Codecov...
    Uploading to S3...
Error: 400 Client Error: Bad Request for url: https://storage.googleapis.com/codecov/v4/raw/2020-05-26/3F10B6B6FEA878CCC9F58EAF87855DA8/714379bfb318ec39566729b2fecce6339ff55daf/17397def-dfec-47bb-b20f-879e223b0e90.txt?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=GOOG1EQX6OZVJGHKK3633AAFGLBUCOOATRACRQRQF6HMSMLYUP6EAD6XSWAAY%2F20200526%2FUS%2Fs3%2Faws4_request&X-Amz-Date=20200526T132703Z&X-Amz-Expires=10&X-Amz-SignedHeaders=host&X-Amz-Signature=86369f603a1c27782899d7df6206b35a4564c70fe4efaea3cb72dd03cfb80b36

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```