Enable usage of custom Clang-Tidy version

[ardabbour]

It would be great to use a custom version of Clang-Tidy, especially given that clang-tidy-10, which is the default version of ubuntu:20.04, raises a clang-diagnostic-error on several actionlib headers. As a workaround, one can do something like this:

variables:
  BEFORE_CLANG_TIDY_CHECKS : |
    if [ "$EUID" -ne 0 ] && command -v sudo > /dev/null; then
      sudo apt-get -qq install -y --no-upgrade --no-install-recommends clang-tidy-12
      sudo ln -sf $(which clang-tidy-12) $(which clang-tidy)
    else
      apt-get -qq install -y --no-upgrade --no-install-recommends clang-tidy-12
      ln -sf $(which clang-tidy-12) $(which clang-tidy)
    fi

But it would waste less CI minutes and overall be cleaner if there was a way to choose the Clang-Tidy version of interest, as is done with Clang-Format.

[mathias-luedtke]

Why don't you use ADDITIONAL_DEBS?
Do other versions not setup set themselves as default?

[ardabbour]

I believe the version used for setup changes depending on the base OS, whatever the default version is gets aliased to the clang-tidy command, which is then used for the checks. Even if the required version is installed via ADDITIONAL_DEBS, the clang-tidy command used later on will trigger the OS's default Clang-Tidy version. Also, installing the default version is a (minor) waste of CI time.