Using the script is a bit simpler if it is added to a location in
the user's PATH with execution permission set. The Makefile will,
in addition, remove the file extension. After running make it is
a simple matter of using `pilot` on the command line.

Pilot recognises some environment variables and uses them if they
are set. This adds some text to the readme so it is clear for the
user.

The queuing system was hard-coded to slurm. It may be, at times,
convenient to be able to run workflows on the local node (or use
a different queuing system altogether).

The old behaviour is maintained by setting the batching system
to slurm by default; the user can change the system by using
--batch_system.

Apptainer v1.3.6-1.el9 doesn't appear to allow resetting the
container path using APPTAINERENV_PATH. Using this variable
works in v1.3.4, but it isn't clear where this change was
introduced. APPTAINERENV_PREPEND_PATH is a safe alternative.

In some versions of toil the `:latest` tag appears to be
automatically appended. This is a workaround for these
versions.