Replace set-env, deprecated in GH actions. Using [GH actions env files] (https://docs.github.com/en/free-pro-team@latest/actions/reference/workflow-commands-for-github-actions#environment-files)

Signed-off-by: Guillaume Tardif <guillaume.tardif@docker.com>

.github/workflows/release.yml

@@ -10,7 +10,7 @@ jobs:
     steps:
       - uses: actions/checkout@v2
       - name: Set env
-        run: echo ::set-env name=RELEASE_VERSION::$(echo ${GITHUB_REF:11})
+        run: echo "RELEASE_VERSION=${GITHUB_REF:11}" >> $GITHUB_ENV
       - uses: actions/setup-node@v1
         with:
           node-version: 12