fix(ci): correct prerelease branch ref from main to master

The repo's default branch is master, not main. The incorrect ref
would prevent the prerelease NuGet publishing job from ever triggering.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: koenbeuk

.github/workflows/build.yml

@@ -65,7 +65,7 @@ jobs:
 
   prerelease:
     needs: build
-    if: github.ref == 'refs/heads/main'
+    if: github.ref == 'refs/heads/master'
     runs-on: ubuntu-latest
     steps:
       - name: Download Artifact