Ensure that the file relative to the example scripts is called.
Previously, the unqualified call would give precedence to source files in
PATH. This could lead to the wrong file being called when dir
`helper` (containing another file named `start-bash-session.sh`) was in PATH.
- The scripts now work within arbitrary nix-shells.
Previously, they failed when run from nix shells other than
`examples/shell.nix`.
- The scripts now work from arbitrary working dirs.
