ci(workflow): add boot sign key setup step

Add conditional step to set boot sign key from secrets if present, required for secure boot image signing

.github/workflows/build.yml

@@ -72,6 +72,14 @@ jobs:
           chmod a+rx ./git-repo/repo
           echo "REPO=$GITHUB_WORKSPACE/./git-repo/repo" >> $GITHUB_ENV
 
+      - name: Set boot sign key
+        env:
+          BOOT_SIGN_KEY: ${{ secrets.BOOT_SIGN_KEY }}
+        run: |
+          if [ ! -z "$BOOT_SIGN_KEY" ]; then
+            echo "$BOOT_SIGN_KEY" > ./kernel-build-tools/linux-x86/share/avb/testkey_rsa2048.pem
+          fi
+
       - name: Clone AnyKernel3 and Other Dependencies
         run: |
           ANYKERNEL_BRANCH="gki-2.0"