ci: Use GITHUB_OUTPUT envvar instead of set-output command (#232)

Summary:
`save-state` and `set-output` commands used in GitHub Actions are deprecated and [GitHub recommends using environment files](https://github.blog/changelog/2023-07-24-github-actions-update-on-save-state-and-set-output-commands/).

This PR updates the usage of `set-output` to `$GITHUB_OUTPUT`

Instructions for envvar usage from GitHub docs:

https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#setting-an-output-parameter

Pull Request resolved: #232

Reviewed By: williamspatrick

Differential Revision: D55439795

fbshipit-source-id: 3146a18887fb91e61f76dfae3b9452ca2a0f9934

.github/workflows/flashy.yml

@@ -55,7 +55,7 @@ jobs:
           cp tools/flashy/scripts/run_flashy*.sh tools/flashy/release/scripts
       - name: Get short SHA
         id: slug
-        run: echo "::set-output name=sha7::$(echo ${GITHUB_SHA} | cut -c1-7)"
+        run: echo "sha7=$(echo ${GITHUB_SHA} | cut -c1-7)" >> $GITHUB_OUTPUT
       - name: Create Release
         id: create_release
         uses: actions/create-release@v1