Fix CI when running against PRs and add a CI stage to test against NS2.0 target

[neon-sunset]

1. Current security permissions prevent the access to the API key for the test env when running against PRs - for productive collaboration it is best to fix that.

2. Now with the addition of NS2.0, there are two major TFMs we care about: latest LTS net8.0 and netstandard2.0. It is important to test against the latter as the code in the dependencies might differ, just like in this library, and needs validation.