Moar current GH Actions syntax for passing env vars around

.github/workflows/nightly.yml

@@ -40,7 +40,7 @@ jobs:
     - name: Add -nightly-(commit ID) prefix to version
       id: set-nightly-version
       run: |
-        echo ::set-env name=NIGHTLY_VERSION::${{ steps.set-version.outputs.version }}-nightly-`date +%F`-`git rev-parse --short HEAD`
+        echo NIGHTLY_VERSION=${{ steps.set-version.outputs.version }}-nightly+`date +%F`-`git rev-parse --short HEAD` >> "$GITHUB_ENV"
 
     - name: Log version & changelog
       run: |