fix(ci): generate coverage XML for manager app so TiCS can ingest it

[cbartz]

What this PR does

Adds coverage xml -o coverage/coverage.xml to the coverage-report tox env in github-runner-manager/tox.ini.

The TiCS workflow runs coverage for both the charm (root tox.ini) and the manager app (github-runner-manager/tox.ini). The root env already produced an XML report, but the manager's coverage-report env only ran coverage report (a terminal summary) and never emitted XML.

Why we need it

TiCS scans the workspace for a coverage XML report to ingest unit-test coverage. With no XML produced for the manager app, TiCS was blocked. This change makes the manager env emit coverage/coverage.xml, matching the root tox.ini pattern.

Verified locally: tox -e unit,coverage-report in github-runner-manager/ now writes a valid Cobertura XML report (86% coverage, above the fail_under=73 threshold).

Checklist

• I followed the contributing guide
• I added or updated the documentation (if applicable) — N/A, CI-only change
• I updated docs/changelog.md with user-relevant changes — N/A, no user-facing change
• I used AI to assist with preparing this PR
• I added or updated tests as needed (unit and integration) — N/A, CI config change
• If this is a Grafana dashboard: I added a screenshot of the dashboard — N/A
• If this is Terraform: terraform fmt passes and tflint reports no errors — N/A
• If the github-runner-manager application has been changed: The application version number is updated in github-runner-manager/pyproject.toml. — N/A, no application code change (tox config only)