MT#64132 Refresh grml-build-trixie docker image

While the t/Dockerfile was properly updated in
latest commit 342ef1a91c,
for usage as deployment-iso-trixie, the
grml-build-trixie one via grml_build/Dockerfile is
still not up2date.

Force updating it, to debug the outdated grml-build-iso run, see
e.g. https://jenkins.mgm.sipwise.com/job/grml-build-iso/175/console

Change-Id: Ie6443640b08fee809e104e372d2509727f699a83

grml_build/Dockerfile

@@ -6,7 +6,7 @@ FROM docker.mgm.sipwise.com/sipwise-trixie:latest
 # is updated with the current date. It will force refresh of all
 # of the base images and things like `apt-get update` won't be using
 # old cached versions when the Dockerfile is built.
-ENV REFRESHED_AT=2026-04-29
+ENV REFRESHED_AT=2026-06-29
 
 # tools for building and testing
 RUN apt-get update && apt-get install --assume-yes --no-install-recommends \