ci: bake cached toolchain into Podman agent and fix nested Buildah [CI]

ci/woodpecker-agent-entrypoint.sh

@@ -0,0 +1,8 @@
+#!/usr/bin/env bash
+set -euo pipefail
+mkdir -p /home/justme/woodpecker/work "$NUGET_PACKAGES" "$NUGET_SCRATCH"
+chmod 700 "$NUGET_SCRATCH"
+# Seed the persistent application restore cache from the baked SDK/Zig packs.
+# Existing package versions are reused; this copies files without downloading.
+cp -a -n /root/.nuget/packages/. "$NUGET_PACKAGES/"
+exec /usr/local/bin/woodpecker-agent "$@"