chore: add e2e test for build-variant and repack-variant build targets

[cezar-r]

Issue number:
#622

Closes #
#622

Description of changes:
Add integration tests for the build-variant and repack-variant make targets in twoliter.

Changes

• Added new test module variant_build.rs with two integration tests:
  • test_twoliter_build_variant() - validates the complete build-variant workflow (update → fetch → build-variant)
  • test_twoliter_repack_variant() - validates the repack-variant workflow (update → fetch → build-variant → repack-variant)

Testing done:

cargo make test

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[cbgbt]

I think I'd rather see these as E2E tests of twoliter commands rather than direct tests of the scripts themselves. We should be running the twoliter command that rebundles an image with certs.