ci: fix JSONDecodeError when combining .coverage.gw* files

For example: https://github.com/trezor/trezor-firmware/actions/runs/12976369614/job/36188913357

Single `.coverage` files were renamed, but `make test_emu_ui_multicore` generates
multiple `.coverage.gw*` files, which may overwrite each other in case there are
multiple similar CI jobs (e.g. when translation-related jobs are enabled).

[no changelog]

.github/actions/upload-coverage/action.yml

@@ -2,7 +2,12 @@ name: 'Upload coverage data'
 runs:
   using: "composite"
   steps:
-    - run: mv core/src/.coverage core/src/.coverage.${{ github.job }}${{ strategy.job-index }} || true
+    # add per-job suffix so the .coverage files won't be overwritten during merge (by `core_coverage_report`)
+    - run: |
+        for F in core/src/.coverage core/src/.coverage.gw*
+        do
+          mv -v $F $F.${{ github.job }}${{ strategy.job-index }} || true
+        done
       shell: sh
     - uses: actions/upload-artifact@v4
       with: