The clippy tests had to be run on nightly previously since it wasn't
available with the stable compiler yet, however this had the potential
to fail a lot since not all nightly builds offer clippy.
Since clippy is now available for stable rust, moving clippy to a stable
build should make sure that the failure rate of the CI job is cut down
to a minimum.
This fixes https://github.com/jwilm/alacritty/issues/2007.
Update the Windows shell settings and comments so they look familiar to
Windows users, but comment out the shell settings for consistency with
Linux and Mac.
Stop checking COMSPEC when the configuration file does not specify which
shell to use, and just default to PowerShell.
Christian Duerr