ci: upload artifacts with v3 — v4 cannot talk to Gitea

Build 18745 produced a working image (8.00 GiB compressed to 972 MiB in 44
seconds, checksum recorded) and then failed on the very last step:

    GHESNotSupportedError: @actions/artifact v2.0.0+, upload-artifact@v4+ and
    download-artifact@v4+ are not currently supported on GHES.

Gitea's artifact backend identifies as GHES, and the v2 artifact client behind
upload-artifact@v4 refuses GHES outright rather than falling back. I chose v4
on the assumption Gitea 1.25 supported it; it does not. v3 works.

Dropping compression-level with it — v3 has no such input, and the image is
already zstd so it would have been a no-op anyway.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XWRjNJMistCy6ngXH5aJLS

.gitea/workflows/build-image.yaml

@@ -114,7 +114,11 @@ jobs:
         if: always()
         run: cat output/*.sha256 2>/dev/null || echo "no images produced"
 
-      - uses: actions/upload-artifact@v4
+      # v3, not v4. Gitea's artifact backend identifies as GHES, and
+      # @actions/artifact v2 (which backs upload-artifact@v4) refuses to talk to
+      # GHES outright — "GHESNotSupportedError". v3 works. The image is already
+      # zstd, so v3 storing it uncompressed costs nothing.
+      - uses: actions/upload-artifact@v3
         if: always()
         with:
           name: fedora-lenovo-yoga-c630
@@ -122,7 +126,6 @@ jobs:
             output/*.img.zst
             output/*.sha256
           retention-days: 14
-          compression-level: 0   # already zstd
           if-no-files-found: warn
 
       - name: Attach to release