Run tests also on Linux in GitHub Actions (#233)

* Initial plan

* Add ubuntu-latest to build matrix to run tests on Linux

Co-authored-by: 304NotModified <5808377+304NotModified@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: 304NotModified <5808377+304NotModified@users.noreply.github.com>

Author: Copilot

.github/workflows/build.yml

@@ -9,7 +9,10 @@ on:
 
 jobs:
   build:
-    runs-on: windows-latest
+    strategy:
+      matrix:
+        os: [windows-latest, ubuntu-latest]
+    runs-on: ${{ matrix.os }}
     permissions:
       contents: read
       id-token: write
@@ -32,21 +35,23 @@ jobs:
         run: dotnet test src/NLog.MailKit.sln --configuration Release --no-build
 
       - name: Pack
+        if: matrix.os == 'windows-latest'
         run: dotnet pack src/NLog.MailKit.sln --no-build --configuration Release --output artifacts
 
       - name: Upload nuget package
+        if: matrix.os == 'windows-latest'
         uses: actions/upload-artifact@v4
         with:
           name: nupkg
           path: artifacts/*.nupkg
 
       - name: NuGet login
-        if: github.ref == 'refs/heads/master'
+        if: github.ref == 'refs/heads/master' && matrix.os == 'windows-latest'
         uses: NuGet/login@v1
         id: login
         with:
           user: ${{ secrets.NUGET_USER }}
 
       - name: Push to NuGet.org
-        if: github.ref == 'refs/heads/master'
+        if: github.ref == 'refs/heads/master' && matrix.os == 'windows-latest'
         run: dotnet nuget push artifacts/*.nupkg --api-key ${{ steps.login.outputs.NUGET_API_KEY }} --source https://api.nuget.org/v3/index.json --skip-duplicate