Fix warning in GitHub actions about deprecated .yaml syntax (#100)

Author: jiridanek

.github/workflows/build.yml

@@ -18,7 +18,7 @@ jobs:
         shell: cmake -P {0}
         run: |
           string(TIMESTAMP current_date "%Y-%m-%d-%H;%M;%S" UTC)
-          message("::set-output name=timestamp::${current_date}")
+          file(APPEND "$ENV{GITHUB_OUTPUT}" "timestamp=${current_date}")
 
       - uses: actions/cache@v3
         with: