ci: Install fixed version of clang-format for linters

Author: MarcoFalke

ci/lint/04_install.sh

@@ -6,6 +6,10 @@
 
 export LC_ALL=C
 
+travis_retry sudo apt update && sudo apt install -y clang-format-9
+sudo update-alternatives --install /usr/bin/clang-format      clang-format      $(which clang-format-9     ) 100
+sudo update-alternatives --install /usr/bin/clang-format-diff clang-format-diff $(which clang-format-diff-9) 100
+
 travis_retry pip3 install codespell==1.17.1
 travis_retry pip3 install flake8==3.8.3
 travis_retry pip3 install yq