ci: switch smoke test to actions/upload-artifact + download-artifact

actions/cache is confirmed broken on this Gitea Actions runner (both
save and restore time out against /_apis/artifactcache/*). Testing
whether the separate artifact-upload API path works instead, as a
fallback basis for the CI restructuring.

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

.gitea/workflows/_cache-smoke-test.yml

@@ -5,36 +5,31 @@ on:
     branches: [ci-cache-speedup]
 
 jobs:
-  cache-write:
+  artifact-write:
     runs-on: ubuntu-latest
     steps:
-      - name: Create cache payload
+      - name: Create artifact payload
         run: |
-          mkdir -p /tmp/cache-smoke
-          echo "hello-${{ github.run_id }}" > /tmp/cache-smoke/data.txt
+          mkdir -p /tmp/artifact-smoke
+          echo "hello-${{ github.run_id }}" > /tmp/artifact-smoke/data.txt
 
-      - uses: actions/cache@v4
+      - uses: actions/upload-artifact@v4
         with:
-          path: /tmp/cache-smoke
-          key: cache-smoke-${{ github.run_id }}
+          name: artifact-smoke-${{ github.run_id }}
+          path: /tmp/artifact-smoke
 
-  cache-read:
+  artifact-read:
     runs-on: ubuntu-latest
-    needs: cache-write
+    needs: artifact-write
     steps:
-      - uses: actions/cache@v4
-        id: restore
+      - uses: actions/download-artifact@v4
         with:
-          path: /tmp/cache-smoke
-          key: cache-smoke-${{ github.run_id }}
+          name: artifact-smoke-${{ github.run_id }}
+          path: /tmp/artifact-smoke
 
-      - name: Verify cache was restored
+      - name: Verify artifact was restored
         run: |
-          if [ "${{ steps.restore.outputs.cache-hit }}" != "true" ]; then
-            echo "cache-hit was not true: '${{ steps.restore.outputs.cache-hit }}'" >&2
-            exit 1
-          fi
-          test -f /tmp/cache-smoke/data.txt
-          cat /tmp/cache-smoke/data.txt
-          test "$(cat /tmp/cache-smoke/data.txt)" = "hello-${{ github.run_id }}"
-          echo "actions/cache works on this runner"
+          test -f /tmp/artifact-smoke/data.txt
+          cat /tmp/artifact-smoke/data.txt
+          test "$(cat /tmp/artifact-smoke/data.txt)" = "hello-${{ github.run_id }}"
+          echo "actions/upload-artifact + download-artifact work on this runner"