mkSymlinkActivationScript: fix check to not miss broken symlinks

flake.nix

@@ -15,7 +15,7 @@
 
     mkSymlinkActivationScript = {target, source}: ''
       if [ -e ${source} ]; then
-        if [ -e ${target} ]; then
+        if [ -L ${target} ] || [ -e ${target} ]; then
           rm -rf ${target}
         fi
         ln -s ${source} ${target}