ci: Add missing --external to podman image prune #28708

This should fix the out-of-space issues seen in CI. For example:

https://cirrus-ci.com/task/6208410429947904?logs=ci#L8613

Error: committing container for step {Env:[FILE_ENV=./ci/test/00_setup_env_native_msan.sh PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin] Command:run Args:[bash -c cd /ci_container_base/ && set -o errexit && source ./ci/test/00_setup_env.sh && ./ci/test/01_base_install.sh] Flags:[] Attrs:map[json:true] Message:RUN bash -c cd /ci_container_base/ && set -o errexit && source ./ci/test/00_setup_env.sh && ./ci/test/01_base_install.sh Original:RUN ["bash", "-c", "cd /ci_container_base/ && set -o errexit && source ./ci/test/00_setup_env.sh && ./ci/test/01_base_install.sh"]}: copying layers and metadata for container "8d882455cc157be6a85d9779b45cacf4dd92a37cfb16fad38213f758a830827d": writing blob: adding layer with blob "sha256:371f657e226fef20f4af6fb88a288dd6248c82c2088daca2d53aaacb51b4303a": processing tar file(write /usr/lib/x86_64-linux-gnu/perl/5.34.0/auto/Unicode/Collate/Collate.so: no space left on device): exit status 1

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utACK fa13f0a5c5d4b9792abf6286c9a8a8f7d0fe80b3

nit: would prefer organizing the comments a bit differently

<details> <summary>git diff on fa13f0a5c5d4b9792abf6286c9a8a8f7d0fe80b3</summary>

diff --git a/ci/test/02_run_container.sh b/ci/test/02_run_container.sh
index 4b9cddaa38..26fd6a3f3b 100755
--- a/ci/test/02_run_container.sh
+++ b/ci/test/02_run_container.sh
@@ -35,13 +35,12 @@ if [ -z "$DANGER_RUN_CI_ON_HOST" ]; then
 
     # Still prune everything in case the filtered pruning doesn't work, or if labels were not set
     # on a previous run. Belt and suspenders approach, should be fine to remove in the future.
-    # Prune external containers (build containers) when using podman.
-    # (--external should be set below in the --filter command as well, when
-    # using podman-docker).
+    # Prune images used by --external containers (e.g. build containers) when using podman.
     echo "Prune all dangling images"
     podman image prune --force --external
   fi
   echo "Prune all dangling $CI_IMAGE_LABEL images"
+  # When using podman-docker, add `--external`
   docker image prune --force --filter "label=$CI_IMAGE_LABEL"
 
   # shellcheck disable=SC2086

</details>

utACK fa6588737714cf26571657fc216552a4291376da

Red CI can be ignored for now