Fix intermittent SIGSEGV (exit code 139) on macOS when running
nix-shell and shebang tests inside the nix sandbox.
The foo, bar, and ruby test scripts were created without shebangs,
which causes intermittent crashes when executed via command
substitution on macOS. Adding proper shebangs resolves the flakiness.
Potentially closes: #13106
(cherry picked from commit 7b3d7eb634)