CI: Allow use of "local" keyword

ash supports this just fine, and the benefits outweigh 100% POSIX
compatibility.

.ci/shellcheck.sh

@@ -21,5 +21,5 @@ sh_files="
 for file in $sh_files; do
 	echo "Test with shellcheck: $file"
 	cd "$DIR/../$(dirname "$file")"
-	shellcheck -e SC1008 -x "$(basename "$file")"
+	shellcheck -e SC1008 -e SC3043 -x "$(basename "$file")"
 done