gnu: haskell-mode: Fix test failure.

gnu: haskell-mode: Fix test failure.

Fixes <https://bugs.gnu.org/31726>.

Thanks to Jack Hill <jackhill@jackhill.us> for exploring different solutions
at <https://debbugs.gnu.org/cgi/bugreport.cgi?bug=31726>.

* gnu/packages/patches/haskell-mode-unused-variables.patch,
gnu/packages/patches/haskell-mode-make-check.patch: New files.
* gnu/local.mk (dist_patch_DATA): Add them.
* gnu/packages/emacs.scm (haskell-mode)[source]: Use them.
[arguments]: Adjust 'pre-build' phase to embed file name.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>