Fix CI: downgrade upload/download-artifact to v3 for Gitea compatibility

All 8 packages built and verified successfully in task 96 — the only
remaining failure was the very last step: actions/upload-artifact@v4
errored with GHESNotSupportedError. Gitea Actions' built-in artifact
storage doesn't implement the newer v2 upload/download API that
upload-artifact@v4/download-artifact@v4 require. Downgraded both (they
must match — v3 and v4 artifacts aren't cross-compatible) to v3, which
uses the older protocol Gitea does support.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.github/workflows/build-packages.yml

@@ -69,6 +69,14 @@ on:
 # installs a pinned, checksum-verified Node.js release directly from
 # nodejs.org — the same style already used for Go/Rust in
 # scripts/ci/setup-slackware-buildenv.sh.
+#
+# upload-artifact is pinned to v3, not v4 — Gitea Actions' built-in
+# artifact storage does not implement the newer v2 upload/download API
+# that actions/upload-artifact@v4 and actions/download-artifact@v4
+# require; uploading fails outright with "GHESNotSupportedError". v3 uses
+# the older, still-supported protocol. release.yml's download-artifact
+# must stay on the matching v3 — the two protocols aren't compatible with
+# each other.
 env:
   SLACKWARE_IMAGE: "vbatts/slackware:15.0"
   NODE_VERSION: "20.20.2"
@@ -143,7 +151,7 @@ jobs:
         run: echo "value=podman-packages-${{ github.sha }}" >> "$GITHUB_OUTPUT"
 
       - name: Upload build artifacts
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v3
         with:
           name: ${{ steps.artifact-name.outputs.value }}
           path: |