apkbuild-lint: fix false negative with custom variables

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

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.