try: use dict.get instead of ternary to avoid xonsh parser bug

ci/build.xsh

@@ -13,8 +13,9 @@ print(f"=== Building flake-hub packages for {ARCH} ===")
 attic = $(nix build --inputs-from . nixpkgs#attic-client --print-out-paths --no-link).strip() + "/bin/attic"
 @(attic) login ci @(ATTIC_SERVER) $ATTIC_TOKEN
 
-# Common packages (all arches) + cross-only packages
-packages = ["hello-world", "geesefs", "xonsh"] + (["attic-client"] if ARCH == "s390x-linux" else [])
+# Packages per arch
+_cross_only = {"s390x-linux": ["attic-client"]}
+packages = ["hello-world", "geesefs", "xonsh"] + _cross_only.get(ARCH, [])
 
 print("Building packages...")
 for pkg in packages: