fix(exec): serve the v4 artifact API so upload-artifact v4 and later work (#1341)

`exec` forwarded the toolkit's ArtifactService calls to an instance it never registered, and its random runtime token held no backend IDs, so `actions/upload-artifact` and `actions/download-artifact` v4 and later failed.

The local artifact server now serves the v4 API, including the single Put Blob uploads docker/build-push-action uses. `exec` gives each workflow its own unsigned JWT runtime token and artifact run, so artifact names conflict and carry over between runs as on GitHub. Artifacts default to a temporary directory instead of the workspace that jobs copy.

Co-authored-by: silverwind <[email protected]>
Co-authored-by: Lunny Xiao <[email protected]>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1341
Reviewed-by: Lunny Xiao <[email protected]>
Reviewed-by: silverwind <[email protected]>
Co-authored-by: bircni <[email protected]>

e2e/testdata/workflows/exec_artifact.yml

@@ -0,0 +1,15 @@
+on: push
+jobs:
+  roundtrip:
+    runs-on: ubuntu-latest
+    steps:
+      - run: mkdir -p out && echo exec-payload > out/data.txt
+      - uses: actions/upload-artifact@v7
+        with:
+          name: exec-artifact
+          path: out/data.txt
+      - uses: actions/download-artifact@v8
+        with:
+          name: exec-artifact
+          path: got
+      - run: grep -q exec-payload got/data.txt