ci: Switch to environment files

Fixes:

"Unable to process command '::set-env name=RELEASE_VERSION::2.3'
successfully.
The `set-env` command is disabled. Please upgrade to using Environment
Files [...]."

To make the next release smoother, hopefully.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

.github/workflows/release.yml

@@ -13,7 +13,7 @@ jobs:
       - name: Check out repo
         uses: actions/checkout@v2
       - name: Get release
-        run: echo ::set-env name=RELEASE_VERSION::${GITHUB_REF#refs/tags/}
+        run: echo "RELEASE_VERSION=${GITHUB_REF#refs/tags/}" >> $GITHUB_ENV
       - name: Login to ghcr.io
         uses: docker/login-action@v1
         with: