chore(build): swap -fexternal-interpreter for --disable-shared on models

-fexternal-interpreter had no effect: crash invariant at 287,686,318.
System ar reads libHSghc-9.10.3-5702.a (287,768,576 bytes) fine, but GHC's
internal readAr fails on the last entry — a bug in GHC's AR parser when
reading content near end-of-file. The call site is mergeObjectFiles during
.so creation, not TH evaluation.

--disable-shared skips the shared library build for inter-hub-models,
preventing GHC from ever calling readAr on libHSghc.a.

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

flake.nix

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