build: reduce code duplication when setting the work path

Currently, the code for defining the path to the exercises directory is
duplicate 4 times.

Add the constants `healed_path` and `work_path`, and use work_path
instead of the duplicated if expression.  Update ZiglingStep to take
`work_path` instead of `use_healed` as argument.

Reduce code length by using `join` instead of `std.fs.path.join` and
replace the use of a slice with a tuple.

Additionally, in case of an error from the `join` function, use @panic
instead of unreachable.

Document why the special branch, when the exercises are healed by the
eowyn script, has been disabled.

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