WIP manual clang7

Author: robUx4

.github/workflows/linux.yaml

@@ -100,7 +100,7 @@ jobs:
       - name: install clang-7
         run: |
           wget -qO - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
-          sudo add-apt-repository --yes --update "deb http://apt.llvm.org/focal/ llvm-toolchain-focal main"
+          # sudo add-apt-repository --yes --update "deb http://apt.llvm.org/focal/ llvm-toolchain-focal main"
           sudo DEBIAN_FRONTEND=noninteractive apt-get install -yq --no-install-recommends clang++-7
 
       - name: list compilers