CI: fix CVE-2020-15228

More details:
https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

Signed-off-by: Josef Schlehofer <pepe.schlehofer@gmail.com>

.github/workflows/multi-arch-test-build.yml

@@ -46,7 +46,7 @@ jobs:
           PACKAGES="${PACKAGES:-vim tmux bmon}"
 
           echo "Building $PACKAGES"
-          echo "::set-env name=PACKAGES::$PACKAGES"
+          echo "PACKAGES=$PACKAGES" >> $GITHUB_ENV
 
       - name: Build
         uses: openwrt/gh-action-sdk@v1