Disable parallel building on AppVeyor

Because running Dialyzer in parallel makes everything fail.

appveyor.yml

@@ -3,5 +3,5 @@ build_script:
 - C:\msys64\usr\bin\bash -lc "pacman --noconfirm -Sy"
 - C:\msys64\usr\bin\bash -lc "pacman --noconfirm -S gcc git make"
 test_script:
-- C:\msys64\usr\bin\bash -lc "cd $APPVEYOR_BUILD_FOLDER && make -j 32 -k check"
-- C:\msys64\usr\bin\bash -lc "cd $APPVEYOR_BUILD_FOLDER && make -j 32 -k check LEGACY=1"
+- C:\msys64\usr\bin\bash -lc "cd $APPVEYOR_BUILD_FOLDER && make -k check"
+- C:\msys64\usr\bin\bash -lc "cd $APPVEYOR_BUILD_FOLDER && make -k check LEGACY=1"