Move us to the new large runners pool

For now this keeps all the configuration identical (AFAICT) but we'll
likely want to play with the specifics to move some of the slower
builders to larger machines and the faster builders to smaller machines,
likely reducing overall usage and improving CI times.

src/ci/github-actions/ci.yml

@@ -74,7 +74,7 @@ x--expand-yaml-anchors--remove:
     env: {}
 
   - &job-linux-xl
-    os: ubuntu-20.04-xl
+    os: ubuntu-20.04-16core-64gb
     <<: *base-job
 
   - &job-macos-xl
@@ -82,7 +82,7 @@ x--expand-yaml-anchors--remove:
     <<: *base-job
 
   - &job-windows-xl
-    os: windows-latest-xl
+    os: windows-2019-8core-32gb
     <<: *base-job
 
   - &job-aarch64-linux