nox: Fix uploading coverage report to codecov

[slimreaper35]

Generate forgotten xml file that codecov used for uploading coverage.

Maintainers will complete the following section

• Commit messages are descriptive enough
• Code coverage from testing does not decrease and new code is covered
• Docs updated (if applicable)
• Docs links in the code are still valid (if docs were updated)

Note: if the contribution is external (not from an organization member), the CI
pipeline will not run automatically. After verifying that the CI is safe to run:

• approve GitHub Actions workflows by clicking a button
• approve the Red Hat Trusted App Pipeline container build by commenting /ok-to-test
  (as is the standard for Pipelines as Code)

[slimreaper35]

FYI: https://app.codecov.io/github/containerbuildsystem/cachi2/commits/slimreaper35%2Fcachi2%3Anox

Sorry for the noise 😮‍💨

[slimreaper35]

/retest