ci: Add env var to add packages to install in rootfs

This can make it more convenient for other projects to reuse these
scripts.

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15891>

.gitlab-ci/container/create-rootfs.sh

@@ -36,6 +36,7 @@ INSTALL_CI_FAIRY_PACKAGES="git
 apt-get -y install --no-install-recommends \
     $ARCH_PACKAGES \
     $INSTALL_CI_FAIRY_PACKAGES \
+    $EXTRA_LOCAL_PACKAGES \
     bash \
     ca-certificates \
     firmware-realtek \