ci: build musl only for arm64 and reuse toolchain cache [CI]

ci/stage-node-image.py

@@ -134,7 +134,7 @@ def normalize_mtimes(directory: Path) -> None:
 def main() -> None:
     parser = argparse.ArgumentParser()
     parser.add_argument("component", choices=tuple(CONTAINERFILES))
-    parser.add_argument("runtime_identifier", choices=("linux-x64", "linux-arm64", "linux-musl-x64", "linux-musl-arm64"))
+    parser.add_argument("runtime_identifier", choices=("linux-x64", "linux-arm64", "linux-musl-arm64"))
     parser.add_argument("destination", type=Path)
     args = parser.parse_args()
     stage(args.component, args.runtime_identifier, args.destination)