Replace deprecated set-env github actions command (#24)

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

.github/workflows/release.yml

@@ -30,7 +30,7 @@ jobs:
             echo "Planned next release version ($NEXT_VERSION) already exists, aborting process"
             exit 1
           fi
-          echo "::set-env name=PIPER_version::$NEXT_VERSION"
+          echo "PIPER_version=$NEXT_VERSION" >> $GITHUB_ENV
 
       - name: Build and push
         run: |