ci: persist NuGet restore cache across Linux builds [CI]

ci/cleanup-build-cache.sh

@@ -1,12 +1,12 @@
 #!/usr/bin/env bash
-# Caller holds its store lock. Keep recent image layers and persistent RUN caches.
+# Caller holds its store lock. Keep recent image layers and the host NuGet cache.
 set -euo pipefail
 : "${CONTAINERS_STORAGE_CONF:?an explicit, locked CI Buildah store is required}"
 # No build is active while the caller holds the exclusive store lock. Remove
 # working containers left by canceled builds before considering old image layers.
 buildah unshare buildah rm --all >/dev/null 2>&1 || true
 buildah rmi localhost/moshiro-agent-binaries:linux-musl-x64 >/dev/null 2>&1 || true
-# prune also deletes RUN cache mounts; remove only expired intermediate layers and versioned application images.
+# Remove only expired intermediate layers and versioned application images.
 mapfile -t expired < <(buildah images --all --no-trunc --format '{{.ID}} {{.Created}} {{.Name}}:{{.Tag}}' | python3 -c '
 import re,sys,time
 cutoff=time.time()-7*86400