fix docker_tag

[bmoon4]

Deprecating save-state and set-output commands

https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

Followed this instruction :)

jobs:
  docker_tag:
    runs-on: ubuntu-latest
    steps:
      - name: Checkout repository
        uses: actions/checkout@v3

      - name: Export Docker Tag (Short SHA)
        id: sha
        run: |-
          echo "git_short_sha=sha-$(git rev-parse --short HEAD)" >> $GITHUB_ENV

      - name: Echo Docker tag
        run: |-
          echo "${{ env.git_short_sha }}"

    outputs:
      docker_tag: ${{ env.git_short_sha }}

Tested in ruggable-token-faucet repo