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/nix/default.nix

@@ -19,7 +19,6 @@ in pkgs.llvmPackages.libcxxStdenv.mkDerivation {
 
   # Abseil's sources need to be symlinked into Nix' sources.
   postUnpack = ''
-    ln -fs ${pkgs.abseil_cpp.drvAttrs.src} nix/abseil_cpp
     ln -fs ${pkgs.glog.drvAttrs.src} nix/glog
   '';
 
@@ -35,6 +34,7 @@ in pkgs.llvmPackages.libcxxStdenv.mkDerivation {
 
  # TODO(tazjin): Some of these might only be required for native inputs
   buildInputs = with pkgs; [
+    abseil_cpp
     aws-s3-cpp
     brotli
     bzip2