fix: stop artifact uploads depending on the cache server reaching Gitea (#1216)

Cache v2 makes the cache server the `ACTIONS_RESULTS_URL` origin, so artifact calls arrived there and were proxied on to Gitea, failing whenever it could not reach the instance.

- Artifact calls are answered with a redirect, so the cache server opens no connection to Gitea. A scheme change or an untrusted instance is still proxied, but there the cache server is the runner itself, which already reaches Gitea.
- Failures answer in twirp, not an empty `502` that clients report as `Unexpected end of JSON input`.
- `cache.v2: false` really points artifacts at Gitea now.
- Cache reservations are bound to the job that made them, so two jobs saving one key cannot commit against each other's upload, and a retry after a lost answer no longer fails a saved entry.
- The toolkit patch, which edits the GitHub-host check out of an action's bundle, was left in the shared checkout where a job running with `runner.patch_actions: false` could inherit it. It is put back after the job's copy.
- `exec` names an origin for the cache v2 it advertises, and masks its runtime token.

Behaviour changes: `no_proxy` no longer exempts `cache.external_server`, and `cache.enabled: false` also stops external registration.

Fixes https://gitea.com/gitea/runner/issues/1208
Fixes https://gitea.com/gitea/runner/issues/1211

Assisted by Claude (Opus 5).

Reviewed-on: https://gitea.com/gitea/runner/pulls/1216
Reviewed-by: bircni <bircni@icloud.com>
Co-authored-by: silverwind <me@silverwind.io>

e2e/testdata/workflows/artifact.yml

@@ -0,0 +1,28 @@
+name: artifact
+on: push
+jobs:
+  upload:
+    runs-on: e2e-artifact
+    steps:
+      - run: |
+          echo "results=$ACTIONS_RESULTS_URL cache=$ACTIONS_CACHE_URL v2=${ACTIONS_CACHE_SERVICE_V2:-unset}"
+          if [ "$ACTIONS_CACHE_SERVICE_V2" = "true" ]; then
+            test "$ACTIONS_RESULTS_URL/" = "$ACTIONS_CACHE_URL"
+          else
+            test "$ACTIONS_RESULTS_URL/" != "$ACTIONS_CACHE_URL"
+          fi
+          mkdir -p out
+          echo "artifact-payload" > out/data.txt
+      - uses: actions/upload-artifact@v7
+        with:
+          name: e2e-artifact-${{ github.run_id }}
+          path: out/data.txt
+  download:
+    needs: upload
+    runs-on: e2e-artifact
+    steps:
+      - uses: actions/download-artifact@v8
+        with:
+          name: e2e-artifact-${{ github.run_id }}
+          path: got
+      - run: grep -q artifact-payload got/data.txt