ci: add cachix to CI build

[gabyx]

Proposed Changes

• Add cachix to CI build.
• Improve credential shit on skopeo, pass temp auth.json file.
• Use ci.bootstrap to bootstrap some tools in the Nix CI container image.

Types of Changes

What types of changes does your code introduce? Put an x in the boxes that
apply

• A bug fix (non-breaking change which fixes an issue).
• A new feature (non-breaking change which adds functionality).
• A breaking change (fix or feature that would cause existing
  functionality to not work as expected).
• A non-productive update (documentation, tooling, etc. if none of the
  other choices apply).

Checklist

Put an x in the boxes that apply. You can also fill these out after creating
the PR. If you're unsure about any of them, don't hesitate to ask. We're here to
help! This is simply a reminder of what we are going to look for before merging
your code.

• I have read the
  CONTRIBUTING
  guidelines.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added the necessary documentation (if appropriate).

Further Comments

[gabyx]

@cmdoret : Still some issues:
cachix/cachix#662