SL-19242: Retrieve and decode Windows signing cert, pass to action.

.github/workflows/build.yaml

@@ -267,8 +267,18 @@ jobs:
     needs: build
     runs-on: windows
     steps:
+      - name: Decode certificate file
+        shell: bash
+        env:
+          SIGNING_CERT_WINDOWS: ${{ secrets.SIGNING_CERT_WINDOWS }}
+        run: |
+          mkdir -p .cert
+          echo "$SIGNING_CERT_WINDOWS" | base64 --decode > .cert/authenticode-cert.pfx
+
       - name: Sign and package Windows viewer
         uses: secondlife/viewer-build-util/sign-pkg-windows@main
+        with:
+          certificate: .cert/authenticode-cert.pfx
 
   sign-and-package-mac:
     needs: build