fix(3p/nix): Link to Abseil built by Nix

This didn't work previously ... but now it does. I think setting the
standard explicitly is what did the trick, but it's slightly unclear
to me why.

Either way this means that Abseil is no longer constantly getting
recompiled when building Nix, which is nice.

Change-Id: I377f7b68bf1ef9045df6a2eee8fdd0c92f243547
Reviewed-on: https://cl.tvl.fyi/c/depot/+/921
Tested-by: BuildkiteCI
Reviewed-by: lukegb <lukegb@tvl.fyi>

third_party/abseil_cpp/default.nix

@@ -7,6 +7,10 @@ in llvmPackages.libcxxStdenv.mkDerivation rec {
   src = ./.;
   nativeBuildInputs = [ cmake ];
 
+  cmakeFlags = [
+    "-DCMAKE_CXX_STANDARD=17"
+  ];
+
   meta = with lib; {
     description = "An open-source collection of C++ code designed to augment the C++ standard library";
     homepage = https://abseil.io/;