ycmd: use boost174

PR #138309 switched the default to 177 and that broke the build.
/cc #151469.

pkgs/top-level/all-packages.nix

···
   ycmd = callPackage ../development/tools/misc/ycmd {
     inherit (darwin.apple_sdk.frameworks) Cocoa;
     python = python3;
+    boost = boost174;
   };
 
   yodl = callPackage ../development/tools/misc/yodl { };