grpc: fix build on aarch64

pkgs/development/libraries/grpc/default.nix

···
     export LD_LIBRARY_PATH=$(pwd)''${LD_LIBRARY_PATH:+:}$LD_LIBRARY_PATH
   '';
 
-  NIX_CFLAGS_COMPILE = lib.optionalString stdenv.cc.isClang "-Wno-error=unknown-warning-option";
+  NIX_CFLAGS_COMPILE = lib.optionalString stdenv.cc.isClang "-Wno-error=unknown-warning-option"
+    + lib.optionalString stdenv.isAarch64 "-Wno-error=format-security";
 
   enableParallelBuilds = true;