[CI] Fix SARIF upload failing when reports directory doesn't exist

The analyzer step has continue-on-error, so when it fails the
src/reports directory is never created, causing upload-sarif to error.
Ensure the directory exists before uploading.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: dbrattli

.github/workflows/build.yml

@@ -274,7 +274,12 @@ jobs:
         run: dotnet msbuild /t:AnalyzeFSharpProject ${{ matrix.project }}
         continue-on-error: true
 
+      - name: Ensure reports directory exists
+        if: success() || failure()
+        run: mkdir -p src/reports
+
       - name: Upload SARIF files
+        if: success() || failure()
         uses: github/codeql-action/upload-sarif@v4
         with:
           sarif_file: src/reports