Address the review comment and update the date.

Author: mordante

libcxx/utils/ci/Dockerfile

@@ -45,8 +45,10 @@ ARG BASE_IMAGE
 FROM $BASE_IMAGE AS builder-base
 
 # Changing this file causes a rebuild of the image in a GitHub action.
-# The date uses the ISO format YYYY-MM-DD.
-RUN echo "Last forced update executed on 2025-04-05."
+# However, it does not cause the CI runners to switch to that image
+# automatically, that must be done by updating the SHA in the Github workflow
+# file. The date uses the ISO format YYYY-MM-DD.
+RUN echo "Last forced update executed on 2025-04-28."
 
 # Make sure apt-get doesn't try to prompt for stuff like our time zone, etc.
 ENV DEBIAN_FRONTEND=noninteractive