The first time you run the script
- defines the latest version of Deno
 - updates itself to replace 
dino@latestwithdeno@with the latest version - deletes the content, leaving only the shebang with fixed Deno version
 
For example, after running, the contents of the script may become like this
#!/usr/bin/env -S pkgx deno@1.42.4 run -A