chore: change to native nix scripts

pkgs/flake/fs-diff/default.nix

@@ -1,5 +1,26 @@
-{ self, name, writeShellApplication }:
+{ name, writeShellApplication, ... }:
 writeShellApplication {
   inherit name;
-  text = builtins.readFile "${self}/files/scripts/${name}.sh";
+  text = ''
+    set -euo pipefail
+
+    OLD_TRANSID=$(sudo btrfs subvolume find-new /mnt/root-blank 9999999)
+    OLD_TRANSID=''${OLD_TRANSID#transid marker was }
+
+    sudo btrfs subvolume find-new "/mnt/root" "$OLD_TRANSID" |
+        sed '$d' |
+        cut -f17- -d' ' |
+        sort |
+        uniq |
+        while read -r path; do
+            path="/$path"
+            if [ -L "$path" ]; then
+                : # The path is a symbolic link, so is probably handled by NixOS already
+            elif [ -d "$path" ]; then
+                : # The path is a directory, ignore
+            else
+                echo "$path"
+            fi
+        done
+  '';
 }