Bump actions runner image used for libc++'s builders.

The builders are not producing warnings about the runner version being
out of date. This is an attempt to fix that.

Author: EricWF

libcxx/utils/ci/docker-compose.yml

@@ -21,7 +21,7 @@ services:
       dockerfile: Dockerfile
       target: actions-builder
       args:
-        BASE_IMAGE: ghcr.io/actions/actions-runner:2.314.1
+        BASE_IMAGE: ghcr.io/actions/actions-runner:2.316.1
         <<: *compiler_versions
   android-buildkite-builder:
     image: ghcr.io/libcxx/android-buildkite-builder:${TAG:-latest}