ci: switch yocto-builder Dockerfile to Ubuntu 26.04

pseudo on oe-core master now wraps open_tree_attr(), which needs
struct mount_attr from the system headers. Ubuntu 22.04 (glibc 2.35)
doesn't provide it, so pseudo-native fails to build:

  error: conflicting types for 'real_open_tree_attr'

Move the builder to Ubuntu 26.04.

Signed-off-by: Andrei Gherzan <andrei@gherzan.com>

.github/workflows/docker-images/yocto-builder/Dockerfile

@@ -2,7 +2,7 @@
 #
 # SPDX-License-Identifier: MIT
 
-FROM ubuntu:22.04
+FROM ubuntu:26.04
 
 ARG DEBIAN_FRONTEND="noninteractive"
 RUN apt-get update -qq
@@ -14,7 +14,7 @@ RUN apt-get install -y eatmydata
 # https://git.yoctoproject.org/poky/tree/documentation/tools/host_packages_scripts/ubuntu_essential.sh
 RUN eatmydata apt-get install -qq -y \
 	build-essential chrpath cpio debianutils diffstat file gawk gcc \
-	git iputils-ping libacl1 liblz4-tool locales python3 python3-git \
+	git iputils-ping libacl1 lz4 locales python3 python3-git \
 	python3-jinja2 python3-pexpect python3-pip python3-subunit socat \
 	texinfo unzip wget xz-utils zstd
 
@@ -27,14 +27,14 @@ ENV LANG en_US.utf8
 RUN eatmydata apt-get clean && rm -rf /var/lib/apt/lists/*
 
 # Have bash as shell
-RUN echo "dash dash/sh boolean false" | debconf-set-selections \
- && dpkg-reconfigure dash
+RUN dpkg-divert --local --rename --add /usr/bin/sh \
+ && ln -s bash /usr/bin/sh
 
 # Fix the resource exhaustion problem on the build infrastructure
 RUN echo 'vm.max_map_count = 4048576' >> /etc/sysctl.conf
 
 # Run under normal user called 'ci'
-RUN useradd --create-home --uid 1000 --shell /usr/bin/bash ci
+RUN userdel -r ubuntu && useradd --create-home --uid 1000 --shell /usr/bin/bash ci
 USER ci
 WORKDIR /home/ci