In order to simplify the code, the heal function is called during the
configuration phase, thus resulting in the function being always called
when the build.zig file is run.
This behavior unfortunately causes a serious issue when the user fix a
broken exercise and, during the next step, the heal function tries to heal
the fixed exercise resulting in GNU patch assuming an attempt to reverse
a patch, waiting for input from the terminal.
Run the heal function from the new HealStep step, so that it is called
only during tests.
Rename the outdir constant to work_path, for consistency with build.zig.
Fixes#272