ADFA-2602: Run the uploader from this workflow's own checkout, and mute Slack by default

The 9.6.1 runs wrote their zips to the bucket root instead of the versioned
prefix, so the versioned URL 404'd while the run reported success.

Cause: the workflow file comes from the dispatched ref, but the repository
content -- scripts included -- comes from inputs.ref. Building the toolchain
branch therefore ran that branch's cloudflare-r2-upload.py, which predates
R2_BUCKET/R2_KEY_PREFIX and ignored both. The tell is that the modified
script's 'to R2 <bucket>/<key>' line never appears in those logs.

Check this workflow's own ref out to .workflow-tools and run the uploader
from there, so the workflow no longer depends on the branch under build
carrying its tooling.

Slack is now behind notify_slack (default false): test runs were posting
links to the team channel, and with the bug above those links were wrong.

Claude-Session: https://claude.ai/code/session_017HGpMsUzZ5wxCfMtDZ2HGP

Author: alome007

.github/workflows/generate_assets.yml

@@ -21,6 +21,11 @@ on:
           - site
           - candidate
           - release
+      notify_slack:
+        description: Post the download links to Slack
+        required: false
+        type: boolean
+        default: false
       candidate_path:
         description: 'GreenGeeks candidate staging path (asset_source: candidate), e.g. tmp/assets/candidates/42'
         required: false
@@ -354,6 +359,12 @@ jobs:
           echo "ASSETS_PATH=$zip_path" >> $GITHUB_OUTPUT
           echo "$(du -h "$zip_path" | cut -f1) $zip_path"
 
+      - name: Checkout this workflow's tooling
+        uses: actions/checkout@v4
+        with:
+          ref: ${{ github.ref }}
+          path: .workflow-tools
+
       - name: Install uv
         uses: astral-sh/setup-uv@v6
 
@@ -366,10 +377,11 @@ jobs:
           R2_BUCKET: ${{ vars.R2_ASSETS_BUCKET || 'apk-repo' }}
           R2_KEY_PREFIX: assets/${{ steps.toolchain.outputs.GRADLE_VERSION }}/
         run: |
-          uv run --with boto3 scripts/cloudflare-r2-upload.py "${{ steps.assets_zip.outputs.ASSETS_PATH }}"
+          uv run --with boto3 .workflow-tools/scripts/cloudflare-r2-upload.py "${{ steps.assets_zip.outputs.ASSETS_PATH }}"
           echo "DOWNLOAD_URL=https://download.appdevforall.org/assets/${{ steps.toolchain.outputs.GRADLE_VERSION }}/assets-${{ matrix.arch }}.zip" >> $GITHUB_OUTPUT
 
       - name: Send Slack notification
+        if: inputs.notify_slack
         env:
           SLACK_WEBHOOK: ${{ secrets.SLACK_WEBHOOK_URL }}
           ASSET_LABEL: ${{ matrix.label }}