github: remove use of ::set-env

This command was disabled due to security risk:

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

Replacement is to use environment file:

https://docs.github.com/en/free-pro-team@latest/actions/reference/workflow-commands-for-github-actions#environment-files

.github/workflows/release.yml

@@ -38,7 +38,7 @@ jobs:
           make $MAKEOPTS -C bricks/cityhub
           make $MAKEOPTS -C bricks/technichub
       - name: Get tag
-        run: echo "::set-env name=GITHUB_TAG::${GITHUB_REF#*refs/tags/}"
+        run: echo "GITHUB_TAG=${GITHUB_REF#*refs/tags/}" >> $GITHUB_ENV
       - name: Create Release
         id: create_release
         uses: actions/create-release@v1