ci: generate releases.json on release event

We are currently using the GitHub API in our setup-buildx-action
to check for latest and tagged releases to make sure they exist
before download. But this requires using a token to avoid
rate-limit. It's fine for public runners but GHES runners don't
have the `github.token` populated automatically. They need to
create a PAT.

This PR will solve this issue by generating and pushing a
`releases.json` file in this repo when we publish a GitHub Release
that will then be fetched through `raw.githubusercontent.com`
endpoint on `setup-buildx-action` repo. This endpoint is better
served for our purpose with 5000 requests per hour compared to the
GitHub API endpoint that is limited to 60 requests per hour (unauth)
and 1000 request per hour when authenticated.

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

.github/workflows/releases-json.yml

@@ -0,0 +1,56 @@
+name: releases-json
+
+on:
+  release:
+    types:
+      - released
+
+env:
+  RELEASES_JSON_PATH: .github/releases.json
+
+jobs:
+  generate:
+    uses: crazy-max/.github/.github/workflows/releases-json.yml@5d21ab1b5b9e9bebca815e4d92b8310b4f8765ae
+    with:
+      repository: docker/buildx
+    secrets: inherit
+
+  releases-json:
+    runs-on: ubuntu-22.04
+    needs:
+      - generate
+    steps:
+      -
+        name: Checkout
+        uses: actions/checkout@v3
+      -
+        name: Create .github/releases.json file
+        uses: actions/github-script@v6
+        with:
+          script: |
+            const fs = require('fs');
+            await fs.writeFileSync('${{ env.RELEASES_JSON_PATH }}', `${{ needs.generate.outputs.releases }}`);
+      -
+        name: Upload releases.json
+        uses: actions/upload-artifact@v3
+        with:
+          name: releases-json
+          path: .github/releases.json
+          if-no-files-found: error
+      -
+        name: Commit changes
+        run: |
+          git add -A .
+      -
+        name: Create PR
+        uses: peter-evans/create-pull-request@2b011faafdcbc9ceb11414d64d0573f37c774b04
+        with:
+          base: master
+          branch: releases-json/${{ github.event.release.name }}
+          commit-message: "update .github/releases.json"
+          signoff: true
+          delete-branch: true
+          title: "Update `.github/releases.json`"
+          body: |
+            Update `.github/releases.json` to keep in sync with GitHub Releases.
+          draft: false