Build pushes on master only; pull requests build every change

[imranakram]

Each push to a branch with an open pull request started two identical CI builds: one for the push, one for the pull request.

The cause

build.yml triggered on push to every branch ('**') and on pull_request to master. A push to a branch with an open pull request matches both.

Both runs build Release and Debug, run all tests and pack the NuGet packages. Each run also counts towards the version, 1.<year>.<month>.<builds this month>, so the duplicates used up version numbers as well.

The change

on:
  push:
    branches:
      - master
  pull_request:
    branches:
      - master
  workflow_dispatch:

• Pushes to master still build. The Release workflow publishes from those runs, so nothing changes there.
• Every change on its way to master is built by its pull request, against the merge result, which is the more useful build of the two.
• workflow_dispatch builds any branch by hand from the Actions tab, for a branch without a pull request.

Only the trigger section of build.yml changes. The build steps and the version calculation are untouched.