c3d2-web: fix permissions in deploy-c3d2-web.service

hosts/containers/c3d2-web/default.nix

@@ -172,6 +172,10 @@ in
         cd c3d2-web
         git fetch origin
         git reset --hard origin/master
+
+        # `make export` may have created read-only files,
+        # fix that before cleaning up
+        chmod -R u+w .
         git clean -d -f -x
       fi