Other than that it looks quite useful, and it's definitely something to keep in the tool belt. Bonus points for the subtle Undertale references too :)
I've started doing this for my Rust projects too because Rust is becoming a really nice language for writing these sorts of things, and having a separate binary as a Rust file in src/bin is convenient.
Often you want to use some logic from the main application in your tasks and using the same language streamlines that.
In a past life, we used invoke [1] for task running. It was incredible but has the same problem as rake: it introduces another language (Python) and more dependencies.
There's a fairly new task runner being developed in go called mage [2], but it didn't seem worth the jump yet to me as it's still pretty immature (I haven't played with it in a few months, though). Did you consider trying that out?
[1] https://github.com/pyinvoke/invoke [2] https://github.com/magefile/mage
Biggest advantage over a generic tool like Make is that you can import your project dependencies (such as your database configuration) into your task.
I like keeping just one language and tooling, but the biggest benefit to me is I'm always forgetting the arcane bits of shell syntax, and using Go saves me from having to search it all the time. Despite being more verbose than bash, I find it saves me time overall.