ci: use new env var syntax

[x1unix]

Recently, GitHub Actions changed a way to declare environment variables which breaks our release process.

This PR fixes this by using a new way to set environment variables.

Before (broken):

      - name: Set release version
        run: echo ::set-env name=RELEASE_VERSION::${GITHUB_REF#refs/*/v}

New way (correct):

      - name: Set release version
        run: echo "RELEASE_VERSION=${GITHUB_REF#refs/*/v}" >> $GITHUB_ENV