github-runner: 2.320.0 -> 2.321.0

* remove mentions of Node.js 16 which is not supported anymore
  see actions/runner#3503
* switch to .Net SDK 8
  see actions/runner#3500
* fix source path discovery in deterministic build: runtime assumes that
  the root of the project is `/_`, replace it with actual `/build/src`
  so that tests can find their data.

(cherry picked from commit db1d5cf)