📌 Pin test-results-action to latest commit

This is a workaround for the Codecov action bug that seems to have a PR
fixing it but isn't yet released:
codecov/test-results-action#108.

Author: webknjaz

.github/workflows/reusable-tox.yml

@@ -391,7 +391,10 @@ jobs:
       if: >-
         !cancelled()
         && steps.tox-run.outputs.test-result-files != ''
-      uses: codecov/test-results-action@v1
+      # FIXME: revert to v1 once Codecov releases v1.0.2 of their action.
+      # Ref: https://github.com/codecov/test-results-action/issues/108.
+      # uses: codecov/test-results-action@v1
+      uses: codecov/test-results-action@4e79e65778be1cecd5df25e14af1eafb6df80ea9
       with:
         disable_search: true
         fail_ci_if_error: >-