eowyn: run `zig fmt --check` before `zig build`

Update the eowyn.sh script to check the healed exercises formatting,
before running `zig build`.

patches/eowyn.sh

@@ -42,5 +42,8 @@ do
     fi
 done
 
+# Check the healed exercises formatting.
+zig fmt --check patches/healed
+
 # Test the healed exercises. May the compiler have mercy upon us.
 zig build -Dhealed