ci: Slim down lint image

Currently, the lint_test_runner is built and installed into the lint CI
image. This is problematic, because it triggers a full image build on
every change to its source code. Doing a build of the lint test_runner
on every run is easier and faster.

ci/lint/06_script.sh

@@ -16,7 +16,7 @@ if [ -n "$CIRRUS_PR" ]; then
   fi
 fi
 
-RUST_BACKTRACE=1 "${LINT_RUNNER_PATH}/test_runner"
+RUST_BACKTRACE=1 cargo run --manifest-path "./test/lint/test_runner/Cargo.toml"
 
 if [ "$CIRRUS_REPO_FULL_NAME" = "bitcoin/bitcoin" ] && [ "$CIRRUS_PR" = "" ] ; then
     # Sanity check only the last few commits to get notified of missing sigs,