ci: Upgrade Ubuntu to 20.04 and don't use remote LLVM

Ubuntu 20.04 has officially replaced 19.10 as the latest version, and is stable enough that people can move to it. The script doesn't change much for that. However we do remove the needless extra builds here and revert to the system installed clang version, which thanks to other pull requests now works fine. For Ubuntu 18.04 that is clang-8, while for Ubuntu 20.04 it is clang-9.

.github/workflows/main.yml

@@ -70,49 +70,39 @@ jobs:
     name: "Ubuntu 64-bit"
     strategy:
       matrix:
-        ubuntu: [ bionic ]
+        ubuntu: [ bionic, focal ]
         compiler: [ gcc, clang ]
-        compiler-version: [ 8, 9, 10 ]
-        exclude:
-          - ubuntu: bionic
-            compiler: gcc
-            compiler-version: 10
-          - ubuntu: bionic
-            compiler: clang
-            compiler-version: 8
         include:
           - ubuntu: bionic
             compiler: gcc
-            compiler-version: 8
             compiler-cxx: g++
             runner: ubuntu-18.04
             packages: gcc-8 g++8
             extra_command: sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-8 800 --slave /usr/bin/g++ g++ /usr/bin/g++-8
-            id: ubuntu1804-gcc8
+            id: ubuntu-18.04
           - ubuntu: bionic
+            compiler: clang
+            compiler-cxx: clang
+            compiler-version: 8
+            runner: ubuntu-18.04
+            packages: clang-8
+            extra_command: ""
+            id: ubuntu-18.04-clang
+          - ubuntu: focal
             compiler: gcc
-            compiler-version: 9
             compiler-cxx: g++
-            runner: ubuntu-latest
+            runner: ubuntu-20.04
             packages: gcc-9 g++9
             extra_command: sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-9 800 --slave /usr/bin/g++ g++ /usr/bin/g++-9
-            id: ubuntu1910-gcc9
+            id: ubuntu-20.04
-          - ubuntu: bionic
+          - ubuntu: focal
             compiler: clang
+            compiler-cxx: clang
             compiler-version: 9
-            compiler-cxx: clang++
+            runner: ubuntu-20.04
-            runner: ubuntu-18.04
             packages: clang-9
             extra_command: ""
-            id: ubuntu1804-clang9
+            id: ubuntu-20.04-clang
-          - ubuntu: bionic
-            compiler: clang
-            compiler-version: 10
-            compiler-cxx: clang++
-            runner: ubuntu-latest
-            packages: clang-10
-            extra_command: ""
-            id: ubuntu1910-clang10
     runs-on: ${{ matrix.runner }}
     env:
       CMAKE_GENERATOR: "Ninja"
@@ -127,10 +117,6 @@ jobs:
     - name: "Prerequisites: Apt-Get"
       shell: bash
       run: |
-        if [ "${{ matrix.compiler }}" == "clang" ]; then
-          wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
-          sudo add-apt-repository "deb http://apt.llvm.org/${{ matrix.ubuntu }}/ llvm-toolchain-${{ matrix.ubuntu }}-${{ matrix.compiler-version }} main"
-        fi
         sudo apt-get -qq update
         sudo apt-get purge libjpeg9-dev:amd64 libjpeg8-dev:amd64 libjpeg-turbo8-dev:amd64
         sudo apt-get install \