fix(nix): only create compile_commands.json if it doesn't exist

Signed-off-by: Seth Flynn <getchoo@tuta.io>

flake.nix

@@ -105,8 +105,10 @@
             ];
 
             shellHook = ''
+              if [ ! -f compile_commands.json ]; then
                 cmake $cmakeFlags -D CMAKE_BUILD_TYPE=Debug
                 ln -s {build/,}compile_commands.json
+              fi
             '';
           };
         }