Install pandoc in GitHub runners

Today while cheking SonarCloud for some code smells I realized that it has not been run in the last month, checking GitHub it fails at:

CMake Error at cmake/Pandocology.cmake:52 (message):
  Pandoc not found.  Install Pandoc (http://johnmacfarlane.net/pandoc/) or
  set cache variable PANDOC_EXECUTABLE.
Call Stack (most recent call first):
  CMakeLists.txt:108 (include)

This error was introduced by commit 9a965d85

We need to install pandoc in the runner.