gha: remove last deprecated set-output

Signed-off-by: clux <sszynrae@gmail.com>

Author: clux

.github/workflows/stable.yml

@@ -50,7 +50,7 @@ jobs:
           pip3 install --user toml
           if python3 check_stable.py; then
             echo "Stable tag missing; running all build steps"
-            echo '::set-output name=BUILD::YES'
+            echo "BUILD=YES" >> $GITHUB_OUTPUT
           else
             echo "Stable tag found; skipping all build steps"
           fi
@@ -159,7 +159,7 @@ jobs:
           pip3 install --user toml
           if python3 check_stable.py; then
             echo "Stable tag missing; running all build steps"
-            echo '::set-output name=BUILD::YES'
+            echo "BUILD=YES" >> $GITHUB_OUTPUT
           else
             echo "Stable tag found; skipping all build steps"
           fi