Fix mkdir permission error for /opt/gh-aw on Linux

[Copilot]

create_dir() in setup.sh only used sudo on macOS, but /opt is root-owned on Linux too. Users who preinstall packages into /opt on ubuntu-latest runners hit mkdir: cannot create directory '/opt/gh-aw': Permission denied.

• Replace OS-detection logic with a try-first approach: attempt mkdir -p without sudo, fall back to sudo only on failure

# Before: only handled macOS
create_dir() {
  if [[ "$(uname -s)" == "Darwin" ]]; then
    sudo mkdir -p "$1"
    sudo chown -R "$(whoami)" "$1"
  else
    mkdir -p "$1"
  fi
}

# After: platform-agnostic, least-privilege
create_dir() {
  if mkdir -p "$1" 2>/dev/null; then
    return
  fi
  sudo mkdir -p "$1"
  sudo chown -R "$(whoami)" "$1"
}

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[pelikhan]

@Mossaka related to #20143 ? We should finish this PR before doing this fix.