[CI] Switch to http://apt.llvm.org/focal/ for installing LLVM.

.github/workflows/ci.yml

@@ -272,7 +272,7 @@ jobs:
         run: |
           curl --retry ${INSTALL_COMPILER_RETRIES} -L https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
           echo 'Acquire::Retries "${INSTALL_COMPILER_RETRIES}";' | sudo tee -a /etc/apt/apt.conf.d/99retries
-          sudo add-apt-repository "deb http://apt.llvm.org/bionic/ llvm-toolchain-bionic-${{ matrix.version }} main"
+          sudo add-apt-repository "deb http://apt.llvm.org/focal/ llvm-toolchain-focal-${{ matrix.version }} main"
           sudo apt-get update
           sudo apt-get install -y clang-${{ matrix.version }} libbsd-dev uuid-dev libidn11
           curl --retry ${INSTALL_COMPILER_RETRIES} -O -L https://github.com/Kitware/CMake/releases/download/v${CMAKE_VERSION}/cmake-${CMAKE_VERSION}-Linux-x86_64.tar.gz