Enable workflow on PR

.github/workflows/main.yml

@@ -1,6 +1,10 @@
 name: main
 
-on: [push]
+on: [push, pull_request]
+
+# Needed for .NET 9 support, as this is currently not included on ubuntu-latest
+env:
+  DOTNET_VERSION: '9.0.x'
 
 jobs:
   build:
@@ -11,6 +15,11 @@ jobs:
         with:
           fetch-depth: 0
 
+      - name: Setup .NET
+        uses: actions/setup-dotnet@v4
+        with:
+          dotnet-version: ${{ env.DOTNET_VERSION }}
+
       - name: Build
         run: |
           dotnet restore dotnet-packaging.sln
@@ -23,6 +32,7 @@ jobs:
           dotnet test Packaging.Targets.Tests/Packaging.Targets.Tests.csproj -l "trx;LogFileName=pkg/Packaging.Targets.Tests.trx"
 
       - uses: actions/upload-artifact@v4
+        if: github.event_name != 'pull_request'
         with:
           name: pkg
           path: pkg/