run shellcheck in local test script (exercism#2038)

`bin/` is the only directory where we have shell scripts. This will fail
if shellcheck is not installed, but even the Ubuntu repos have it, so
nobody should have trouble installing it.

Author: senekor

justfile

@@ -17,7 +17,7 @@ uuid:
 test:
     just configlet lint
     ./bin/lint_markdown.sh
-    # TODO shellcheck
+    shellcheck bin/*.sh
     ./bin/check_exercises.sh
     CLIPPY=true ./bin/check_exercises.sh
     cd rust-tooling && cargo test