The smallest footprint for an actual scripting probably will be posix sh - since you already have it ready.
A slightly bigger footprint would be Python or Lua.
If you can drop your requirement for actual scripting and are willing to add a compile step, Go and it’s ecosystem is pretty dang powerful and it’s really easy to learn for small automation tasks.
Personally, with the requirement of not adding too much space for runtimes, I’d write it in go. You don’t need a runtime, you can compile it to a really small zero dependency lib and you have clean and readable code that you can extend, test and maintain easily.
This is the winner. Combine that with a vastly bigger group of inexperienced developers (and I’m willing to die on that hill), and you have a lot of people running node / npm as an admin / root user, who have close to zero idea what they are doing, hitting their project with third party dependencies left and right for no particular reason (left-pad, is-number, ansi console and similar useless crap), and then your dependency management allows for code execution. Also, from my personal feeling, it seems that npm simply cannot properly audit the packages due to the sheer mass. From a technical standpoint it’s close to trivial to put your malware onto npm, and then you just need to get someone to install your package, which is way simpler than in other package managers