ci: migrate from systemd-boot to systemd-utils in Gentoo container

See https://www-cdn.gentoo.org/support/news-items/2022-04-19-systemd-utils.html

test/container/Dockerfile-Gentoo

@@ -7,9 +7,9 @@ COPY --from=portage /var/db/repos/gentoo /var/db/repos/gentoo
 
 # systemd-boot
 RUN mkdir -p /etc/portage/package.accept_keywords && \
-    echo "sys-boot/systemd-boot" >> /etc/portage/package.accept_keywords/systemd-boot && \
+    echo "sys-boot/systemd-utils" >> /etc/portage/package.accept_keywords/systemd-boot && \
     echo '>=sys-apps/systemd-utils-251.10 boot' > /etc/portage/package.use/systemd-boot && \
-    emerge -qv sys-boot/systemd-boot
+    emerge -qv sys-boot/systemd-utils
 
 # kernel and its dependencies in a separate builder
 FROM docker.io/gentoo/stage3:$TAG as kernel