Setup fails if `sudo` is not available

Example: in a devcontainer that doesn't have `sudo` installed:

```
[2023 ms] Prompting for sudo password...

[2036 ms] ./setup.sh: line 11: sudo: command not found

[2037 ms] Obtaining sudo credentials failed.

[2037 ms] Exit code 1
[676 ms] Stop: Run in container: # Clone & install dotfiles
```