fix(3p/nix): Fix build of derivation if cmake is present

cmake automatically runs a configure hook which breaks the build,
since this isn't actually a cmake project. This hook is now disabled.

Additionally Abseil's sources are linked to an absolute derivation
path when the build launches, as opposed to the relative path used for
development builds.

third_party/nix/default.nix

@@ -18,6 +18,11 @@ in stdenv.mkDerivation {
   version = "2.3.4";
   src = ./.;
 
+  # Abseil's sources need to be linked into a subproject.
+  postUnpack = ''
+    ln -fs ${pkgs.abseil_cpp.src} nix/subprojects/abseil_cpp
+  '';
+
   nativeBuildInputs = with pkgs; [
     bison
     clang-tools
@@ -51,6 +56,9 @@ in stdenv.mkDerivation {
     "-Dsandbox_shell=${pkgs.busybox-sandbox-shell}/bin/busybox"
   ];
 
+  # cmake is only included to build Abseil and its hook should not run
+  dontUseCmakeConfigure = true;
+
   # Install the various symlinks to the Nix binary which users expect
   # to exist.
   postInstall = ''