nix: Use -split-sections with pkgsStatic to make static executable smaller again

This works around https://github.com/NixOS/nixpkgs/issues/286285 to use -split-sections
in a cross-compiling scenario. This will reduce the size of the static executable and also
remove the remaining references to /nix/store/.. reducing closure size dramatically.

This also fixes the docker image blowing up in size since we switched to pkgsStatic.

default.nix

@@ -28,6 +28,12 @@ let
       sha256 = nixpkgsVersion.tarballHash;
     };
 
+  nixpkgs-patched = (import nixpkgs { inherit overlays system; }).applyPatches {
+    name = "nixpkgs-patched";
+    src = nixpkgs;
+    patches = [ nix/split-sections-cross.patch ];
+  };
+
   allOverlays =
     import nix/overlays;
 
@@ -46,7 +52,7 @@ let
 
   # Evaluated expression of the Nixpkgs repository.
   pkgs =
-    import nixpkgs { inherit overlays system; };
+    import nixpkgs-patched { inherit overlays system; };
 
   postgresqlVersions =
     [
@@ -82,7 +88,7 @@ let
   inherit (pkgs.haskell) lib;
 in
 rec {
-  inherit nixpkgs pkgs;
+  inherit nixpkgs-patched pkgs;
 
   # Derivation for the PostgREST Haskell package, including the executable,
   # libraries and documentation. We disable running the test suite on Nix

nix/split-sections-cross.patch

@@ -0,0 +1,14 @@
+This works around https://github.com/NixOS/nixpkgs/issues/286285 by passing --enable-split-sections instead of ghc-options.
+---
+--- a/pkgs/development/haskell-modules/generic-builder.nix
++++ b/pkgs/development/haskell-modules/generic-builder.nix
+@@ -245,8 +245,7 @@ let
+     (enableFeature doBenchmark "benchmarks")
+     "--enable-library-vanilla"  # TODO: Should this be configurable?
+     (enableFeature enableLibraryForGhci "library-for-ghci")
+-  ] ++ optionals (enableDeadCodeElimination && (lib.versionOlder "8.0.1" ghc.version)) [
+-     "--ghc-option=-split-sections"
++    (enableFeature enableDeadCodeElimination "split-sections")
+   ] ++ optionals dontStrip [
+     "--disable-library-stripping"
+     "--disable-executable-stripping"