chore(debug): force -j1 to serialize GHC parallel code generation

With -j8, GHC generates code for 8 modules in parallel. A parallel merging
step might read combined objects via Data.Binary.Get hitting 287 MB. Forcing
-j1 serializes codegen to test if parallel merging is the crash cause.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

flake.nix

@@ -125,7 +125,7 @@
                                                "--ghc-option=-O0"
                                                "--ghc-option=-fomit-interface-pragmas"
                                                "--disable-split-sections"
-                                               "--ghc-option=-fno-dynamic-too"
+                                               "--ghc-option=-j1"
                                            ];
                                            postUnpack = (old.postUnpack or "") + ''
                                                _actual="$sourceRoot/build/Generated/ActualTypes.hs"