install.sh: script broken

Install test failed:

https://github.com/mmcloughlin/addchain/actions/runs/4030515351/jobs/6929229953

Running locally:

```
mbm@xps ~/D/addchain (master)> ./install.sh
mmcloughlin/addchain info checking GitHub for latest tag
mmcloughlin/addchain info found version:  <!DOCTYPE html> <html lang= for  <!DOCTYPE html> <html lang=/linux/amd64
```