Document or remove dependency on bash

### Checklist

- [X] My issue is specific & actionable.
- [X] I am not suggesting a protocol enhancement.
- [X] I have searched on the [issue tracker](https://github.com/ipfs/kubo/issues?q=is%3Aissue) for my issue.

### Description

The [Build from Source](https://github.com/ipfs/kubo#build-from-source) requirements do not mention `bash`, yet the build process depends on it:

```
~/kubo $ make build
go version go1.20.2 linux/amd64
bin/check_go_version 1.18
plugin/loader/preload.sh > plugin/loader/preload.go
env: can't execute 'bash': No such file or directory
make: *** [plugin/loader/Rules.mk:8: plugin/loader/preload.go] Error 127
~/kubo $
```

It would be nice to either use `/bin/sh` or at least document that `bash` is needed.