Pin upload-artifact to v3: v4 refuses to run against Gitea

The env-list fix got every test green on the runner for the first time --
37 of 37, the JUnit publish included -- and the job then failed in the
artifact upload: upload-artifact@v4 speaks GitHub's v2 artifact service and
hard-refuses any host that is not github.com ("not currently supported on
GHES"). Gitea implements the v3 artifact API, so the three upload steps --
coverage, the benchmark tables, the valgrind logs -- pin to v3.

Co-Authored-By: Claude Code (Claude Fable 5, claude-fable-5) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KzBDV2fqgnUAcqCKqKvc71

.gitea/workflows/ci.yaml

@@ -56,7 +56,7 @@ jobs:
           fail_on_failure: 'true'
       - name: Upload coverage reports
         if: always()
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v3
         with:
           name: code-coverage
           path: build/coverage/
@@ -123,7 +123,7 @@ jobs:
       # the next baseline if somebody re-records PERFORMANCE.md.
       - name: Upload benchmark tables
         if: always()
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v3
         with:
           name: performance-baseline
           path: perf-baseline.txt
@@ -170,7 +170,7 @@ jobs:
         run: scripts/memcheck.sh
       - name: Upload valgrind logs
         if: always()
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v3
         with:
           name: valgrind-logs
           path: build/Testing/Temporary/MemoryChecker.*.log