GitHub deprecated the `set-output` (and `set-state`, which we did not
use) commands in Actions scripts due to trust issues.
This patch moves the output management to the new format involving
_"Environment files"_.
The image contents of `ubuntu-20.04` changed sometime in April or May
and they already come with a `clang-tidy` pre-installed and registered
into the alternatives system, which broke the action's install script.
