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.
The general style for errors, warnings and info messages is to start
with a capitalized letter and end without a period. The main exception
is when dealing with nouns that are clearer with special case handling,
e.g. "macOS failed to work" or "ioctl is borked".
David Hewitt