swift: use `aarch64` instead of `arm64` on Linux

    else

      targetPlatform.parsed.kernel.name;

  swiftArch = stdenv.hostPlatform.darwinArch;

  # This causes swiftPackages.XCTest to fail to build on aarch64-linux

  # as I believe this is because Apple calls the architecture aarch64

  # on Linux rather than arm64 when used with macOS.

  swiftArch =

    if hostPlatform.isDarwin then hostPlatform.darwinArch else targetPlatform.parsed.cpu.name;

  # On Darwin, a `.swiftmodule` is a subdirectory in `lib/swift/<OS>`,

  # containing binaries for supported archs. On other platforms, binaries are