WIP manual clang7

Author: robUx4

.github/workflows/linux.yaml

@@ -95,10 +95,13 @@ jobs:
     env:
       CMAKE_OPTIONS: -DDEV_MODE=ON -DBUILD_TESTING=ON
       CMAKE_MATROSKA_OPTIONS: -DBUILD_EXAMPLES=ON
+      CXX: clang++-7
     steps:
-      - uses: egor-tensin/setup-clang@v1
-        with:
-          version: 7
+      - 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 DEBIAN_FRONTEND=noninteractive apt-get install -yq --no-install-recommends clang++-7
 
       - name: list compilers
         run: dpkg --list | grep compiler