[in-proc backport] Add <clear /> to nuget.config

[jviau]

Issue describing the changes in this PR

Backports a small change to in-proc branch. These changes are present in dev. I didn't cherry-pick as we have squash commits anyways and it is a 1 line change.

Pull request checklist

IMPORTANT: Currently, changes must be backported to the in-proc branch to be included in Core Tools and non-Flex deployments.

• Backporting to the in-proc branch is not required
  • Otherwise: Link to backporting PR
• My changes do not require documentation changes
  • Otherwise: Documentation issue linked to PR
• My changes should not be added to the release notes for the next release
  • Otherwise: I've added my notes to release_notes.md
• My changes do not need to be backported to a previous version
  • Otherwise: Backport tracked by issue/PR #issue_or_pr
• My changes do not require diagnostic events changes
  • Otherwise: I have added/updated all related diagnostic events and their documentation (Documentation issue linked to PR)
• I have added all required tests (Unit tests, E2E tests)

[fabiocav]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).