rake-compiler-dock images now make a distinction between GNU and
musl builds. Add a `-gnu` extension to `RUBY_TARGET` to make
it possible to run:

```
rb-sys-dock --platform x86_64-linux-gnu --directory . --build
```

Closes #510

* Revert "Append -gnu to RUBY_TARGET for GNU Linux builds (#511)"

This reverts commit 798fad1.

* Make it possible to override RUBY_TARGET

Previously if you attempt to run `rb-sys-dock --platform x86_64-linux-gnu`
the `RUBY_TARGET` was hard-coded to `x86_64-linux`. Now this can
be overriden by the `rb-sys-dock` so that the `--platform` argument
can work with `x86_64-linux` or `x86_64-linux-gnu`.

Previously RUBY_TARGET was never passed along because `sudo -u` was
run. Add this variable to the sudoers file to preserve this varaible.