Incorrect command order in `build-llvm-11.sh`

[jf-li00]

I've noticed the GPG key for the LLVM repository is added after running sudo apt update. This should be done before sudo apt update to avoid warnings about unauthenticated sources.

Here's a corrected version of this script:

#!/bin/bash

echo deb http://apt.llvm.org/bionic/ llvm-toolchain-bionic-11 main | sudo tee -a /etc/apt/sources.list
echo deb-src http://apt.llvm.org/bionic/ llvm-toolchain-bionic-11 main | sudo tee -a /etc/apt/sources.list
wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
sudo apt update
sudo apt upgrade
sudo apt install clang-11 llvm-11 llvm-11-dev lldb-11 lld-11 -y
sudo apt install libc++-11-dev libc++abi-11-dev -y