Update docs-build to c7i

This updates the docs-build nightly configuration to match other uses
of the _linux-build.yml workflow using `runner_prefix` rather than
`runner` directly. The default runner defined in _linux-build.yml is
the c7i variant so this also updates the runner appropriately.

Relates to pytorch/test-infra#7175. While moving to c7i costs 5% more,
CPU intensive jobs should run roughly 15-20% faster resulting in a cost
reduection of 10-15% for those jobs.

Signed-off-by: Thanh Ha <thanh.ha@linuxfoundation.org>

.github/workflows/nightly.yml

@@ -41,7 +41,7 @@ jobs:
     uses: ./.github/workflows/_linux-build.yml
     needs: get-label-type
     with:
-      runner: "${{ needs.get-label-type.outputs.label-type }}linux.2xlarge"
+      runner_prefix: "${{ needs.get-label-type.outputs.label-type }}"
       build-environment: linux-jammy-py3.10-gcc11
       docker-image-name: ci-image:pytorch-linux-jammy-py3.10-gcc11
     secrets: inherit