cbz2ebook: Remove dependency on my zshrc

This script could now easily be ported to bash (or even sh), but we are
using nix-shell anyway, so why not use zsh.

cbz2ebook.sh

@@ -1,5 +1,5 @@
 #!/usr/bin/env nix-shell
-#!nix-shell -i "zsh -i" -p imagemagick unzip zip zsh
+#!nix-shell -i zsh -p imagemagick unzip zip zsh
 set -e
 
 size="1440x1920" # Kobo Forma
@@ -19,7 +19,14 @@ trap cleanup EXIT INT SIGTERM
 
 cd "$tmpdir"
 unzip "$infile"
-renumber 4 **/*.???
+
+# rename files to have a plain 4 digit filename
+i=1
+for file in **/*.???; do
+    mv -n "$file" "$(dirname $file)/$(printf %04d $i).${file##*.}"
+    i=$((i+1))
+done
+
 length=$(ls -1 **/*.??? | wc -l)
 position=0
 for image in **/*.???; do