ci: pin upload-artifact back to v4 (act compat)

actions/upload-artifact v5+ sends a `mime_type` protobuf field that
act's built-in artifact server cannot decode (nektos/act#6022).
The fix PR (nektos/act#6053) is still unmerged, so pin to v4 — the
last major that works under local act — until upstream lands the fix.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: bkero

.github/workflows/ci.yml

@@ -56,7 +56,7 @@ jobs:
           awk -v c="$TOTAL" 'BEGIN { if (c+0 < 80) { print "FAIL: coverage " c "% is below the 80% threshold"; exit 1 } }'
 
       - name: Upload coverage report
-        uses: actions/upload-artifact@v7
+        uses: actions/upload-artifact@v4
         with:
           name: coverage
           path: coverage.out