ci/shellcheck: ignore SC3043 warnings about use of 'local' (MR 4340)

This configures shellcheck in CI to ignore the use of the 'local'. Using
'local' to support variable scoping can prevent a lot of nasty bugs in
shell scripts.
Ash supports 'local', even though it's not a POSIX sh thing. It's used
extensively in our init.sh script, and many Alpine scripts use it.

.ci/shellcheck.sh

@@ -48,5 +48,5 @@ 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