CI: run GitHub Actions on self-hosted arc-runner-set

The org IP allow list blocks GitHub-hosted runner IPs (checkout 403), so
point all jobs at the self-hosted arc-runner-set. Drop the rsync dependency
in build-site.sh (cp + bytecode prune; the ARC runner image has no rsync)
and remove the migrations-mysql job - ARC/Kubernetes has no service
containers, so that MySQL 8 coverage stays on the internal CI.

scripts/build-site.sh

@@ -57,12 +57,15 @@ echo "==> Building frontend (SITE_PLUGINS=$CLOSURE) ..."
 echo "==> Staging backend into $OUT ..."
 rm -rf "$OUT"
 mkdir -p "$OUT/plugins"
-rsync -a --exclude '__pycache__' --exclude '*.pyc' "$REPO/shopdb" "$OUT/"
+# cp (not rsync) so a minimal runner/deploy box without rsync can stage;
+# bytecode is pruned afterwards to match the old --exclude filters.
+cp -a "$REPO/shopdb" "$OUT/"
 for name in ${CLOSURE//,/ }; do
-  rsync -a --exclude '__pycache__' --exclude '*.pyc' \
-    "$REPO/plugins/$name" "$OUT/plugins/"
+  cp -a "$REPO/plugins/$name" "$OUT/plugins/"
 done
 cp -r "$REPO/frontend/dist" "$OUT/frontend-dist"
+find "$OUT" -type d -name '__pycache__' -prune -exec rm -rf {} +
+find "$OUT" -type f -name '*.pyc' -delete
 
 echo ""
 echo "Lean site staged at: $OUT"