[ci] Pinning integration tests

This commit pins the integration tests. This is done in the .integration.nix
file, which in turns uses the derivations from the 'argotest'
repository. These pin argopkgs - which itself pins nixpkgs, so
everything should be completely reproducible.

The integration target provisions the dependencies using nix-shell and
runs the basic 'hello world' container test.
3 jobs for 25-integration-test-version-pinning in 2 minutes and 10 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #16256
integration
integration.test

00:00:11

passed #16255
rapl
py.test

00:01:07

 
  Style
passed #16257
flake8

00:01:03