Use ubuntu-18.04 instead of ubuntu-20.04 for Clang-Tidy

Use ubuntu-18.04 instead of ubuntu-20.04 for Clang-Tidy

The ubuntu-20.04 VM has intermittent build errors that look like this:

  sudo apt-get install libzstd-dev clang-tidy
  [...]
  The following packages have unmet dependencies:
   clang-tidy : Depends: clang-tidy-10 (>= 10~) but it is not going to be installed
  E: Unable to correct problems, you have held broken packages.

Clang-Tidy 9 also found “function ... has a definition with different
parameter names” warnings that needed to be fixed as part of this.

As discussed in #670.