By default, lintcheck will use the `clippy.toml` file found at the
toplevel of the repository (`CARGO_MANIFEST_DIR`). This file is meant
for configuration of Clippy applied to Clippy sources.
This creates a new `lintcheck/ci-config/clippy.toml` file which is used
by the CI when running lintcheck. By default this uses the default
Clippy configuration.