harmonize gh-actions

[earloc]

Currently, there are two build-yamls, which only differ in their trigger:

main.yaml

on:
  push:
    branches: [ main ]

pullrequests.yaml

on:
  pull_request:
    branches: [ main ]

this should be harmonized somehow:

• restore packages
• build
• test
• upload coverage-report

Later, there should also be some steps/actions for pushing everything into NuGet, but that is probably a new gh-actions?!

[earloc]

Turned out that the needed builds for PRs and releases vary too much and that there is currently no need for a harmonized approach.

see #8