Update code-coverage.yml

[fridgepoet]

Motivation: pull_request is actually describing the target branch. So the check for the name of the PR branch is handled in if: !startsWith(github.head_ref, 'dependabot')
See the doc page here:
https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows

To run a job based on the pull request's head branch name (as opposed to the pull request's base branch name), use the github.head_ref context in a conditional.

Description:This change makes it so that any PR branch which starts with dependabot skips the Code Coverage workflow.

Part of grafana/code-coverage#17

[github-actions]

Backend code coverage report for PR #172
No changes

[github-actions]

Frontend code coverage report for PR #172
No changes

[github-actions]

Levitate is-compatible report:

🔍 Resolving @grafana/data@latest...
🔍 Resolving @grafana/ui@latest...
🔍 Resolving @grafana/runtime@latest...
🔍 Resolving @grafana/e2e-selectors@latest...

🔬 Checking compatibility between ./src/module.ts and @grafana/data@9.3.6...
✔ Found @grafana/data version 8.4.11 locally

🔬 Checking compatibility between ./src/module.ts and @grafana/ui@9.3.6...
✔ Found @grafana/ui version 8.4.11 locally

🔬 Checking compatibility between ./src/module.ts and @grafana/runtime@9.3.6...
✔ Found @grafana/runtime version 8.4.11 locally

🔬 Checking compatibility between ./src/module.ts and @grafana/e2e-selectors@9.3.6...
✔ Found @grafana/e2e-selectors version 8.4.11 locally

✔️ ./src/module.ts appears to be compatible with @grafana/data,@grafana/ui,@grafana/runtime,@grafana/e2e-selectors