MT#62249 Update grml-build-trixie docker image

Ensure we're building on top of sipwise-trixie docker image, quoting
from jenkins-configs.git:

  commit c3abc88488284a52f555223ad4e927d56193d561 (HEAD -> master, origin/master, origin/HEAD)
  Author: Michael Prokop <mprokop@sipwise.com>
  Date:   Fri Jan 30 18:47:17 2026 +0100

      MT#62249 Update sipwise-trixie docker image

      Otherwise we end up with shipping ngcp-archive-keyring v2016.01.12+0~mr13.4.1.1
      instead of the current one, v2016.01.12+0~mr14.0.1.2.

      We ended up here after a many-hour debugging session, tackling the SHA-1
      key situation for our Debian repository signing key.

      When updating our stack, everything was fine, until we faked the date of
      our deployment system and ended up with a breaking installation, because
      the expected keyring files weren't there.

      As it turned out, we're using the sipwise-trixie docker image as a base
      for further docker runs, like present in deployment-iso's
      grml-build-trixie one, which we use for building our Grml ISO.

      Make sure we refresh our docker image, to have the latest
      ngcp-archive-keyring package present.

Change-Id: I177c1ef189885c0d9524687d699b200d44626cd7

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=2025-12-15
+ENV REFRESHED_AT=2026-01-30
 
 # tools for building and testing
 RUN apt-get update && apt-get install --assume-yes --no-install-recommends \

t/Dockerfile

@@ -5,7 +5,7 @@ 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=2025-07-22
+ENV REFRESHED_AT=2026-01-30
 
 RUN apt-get update && apt-get install --assume-yes git make gcc dpkg-dev