Plugin should always use path.join to handle working directories that end in '/'

[dbaeumer]

Problematic code is here: https://github.com/MelvinVermeer/eslint-plugin-no-relative-import-paths/blob/main/index.js#LL4

Consider a case where context.getCwd() returns a path that ends in '/' then the statement will result in a path with '//' in it. This can easily be avoided by using path.join() instead.