chore(build): try -fexternal-interpreter to bypass internal static linker

GHC crashes at byte 287,686,318 reading libHSghc-9.10.3.a (~274 MB) via
its internal static linker during TH evaluation of WidgetVersionInclude.
-fexternal-interpreter delegates TH to a separate iserv process using the
dynamic linker, bypassing readAr and the 274 MB archive entirely.

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

flake.nix

@@ -126,6 +126,7 @@
                                                "--ghc-option=-fomit-interface-pragmas"
                                                "--disable-split-sections"
                                                "--ghc-option=-j1"
+                                               "--ghc-option=-fexternal-interpreter"
                                            ];
                                            postUnpack = (old.postUnpack or "") + ''
                                                _actual="$sourceRoot/build/Generated/ActualTypes.hs"