image: chmod u+w before overriding angie.conf

cp from ${angie}/conf preserves read-only permissions from the nix
store, so cp -f failed when overlaying the custom main config.

flake.nix

@@ -61,7 +61,8 @@
           # scgi_params, uwsgi_params, koi-utf, koi-win, etc.) — chart configs
           # `include fastcgi_params;` and similar resolve relative to /etc/angie.
           cp ${angie}/conf/* etc/angie/
-          cp ${mainConf} etc/angie/angie.conf
+          chmod -R u+w etc/angie
+          cp -f ${mainConf} etc/angie/angie.conf
         '';
         config = {
           Entrypoint = [ "${angie}/bin/angie" ];