CI: tag builds fail

https://github.com/christopherfujino/slothscript/actions/runs/22247750815/job/64365228212?pr=64

```
++ echo refs/tags/v0.1.0
++ sed -n 's/refs\/heads\/\(.*\)/\1/p'
+ REF=
+ git clone https://github.com/christopherfujino/slothscript -b ''
Cloning into 'slothscript'...
fatal: Remote branch  not found in upstream origin
Error: Process completed with exit code 128.
```