ci: resume publishing nightly builds from master (dafny-lang#4991)

### Description

This PR resumes publishing nightly builds from `master`. (It was
accidentally removed in dafny-lang#4931.)

<small>By submitting this pull request, I confirm that my contribution
is made under the terms of the [MIT
license](https://github.com/dafny-lang/dafny/blob/master/LICENSE.txt).</small>

.github/workflows/nightly-build.yml

@@ -28,5 +28,6 @@ jobs:
     uses: ./.github/workflows/nightly-build-reusable.yml
     with:
       ref: master
+      publish-prerelease: true
     secrets:
       nuget_api_key: ${{ secrets.NUGET_API_KEY }}