When a non-standard variable has a prefix of a standard variable,
apkbuild-lint does not complain at the moment. For example:

```sh
depends_foo="abc"
```

is a non-standard variable, but because it starts with `depends`, which
is a standard variable, it is not caught.

Let's fix that by including an `=` in the negative lookahead, so no
longer functions as a prefix match. See [regexp101 example][0].

[0]:https://regex101.com/r/qhVwsP/1