Enabling AVX will allow to test future updates that will use these
instructions.
Since MSVC doesn't allow to enable instruction sets for the current CPU,
we enable AVX unconditionally there, in hope that AppVeyor CI runners
support AVX.
Also, different clang versions exhibit various issues in GitHub Actions
when "native" instruction set is enabled, so we also explicitly specify
the instruction set we want for GHA.
In Ubuntu 20.04 there appeared an updated version of the
software-properties-common package in focal-updates, which ships a newer
apt-add-repository version that doesn't support -P/-S/-U command line arguments.
Since we cannot rely on package version checks to determine apt-add-repository
capabilities, we have to parse its --help output instead.
Also, made source list processing more protected against spaces.
