Save the bazel execution log binary in Github Actions

This file can be parsed as per
https://docs.bazel.build/versions/4.1.0/remote-execution-caching-debug.html#comparing-the-execution-logs
to better understand things like cache hits and misses

Author: HoloRin

.github/workflows/test.yaml

@@ -33,7 +33,13 @@ jobs:
         bazelisk test //... \
           --config=rbe-${{ matrix.erlang_version }} \
           --test_tag_filters=-exclusive,-aws,-mixed-version-cluster \
+          --execution_log_binary_file=/tmp/exec.log \
           --verbose_failures
+    - name: SAVE EXECUTION LOG BINARY
+      uses: actions/upload-artifact@v2-preview
+      with:
+        name: execution-log-binary-${{matrix.erlang_version}}.log
+        path: /tmp/exec.log
   test-exclusive:
     name: Test (Exclusive Tests)
     runs-on: ubuntu-18.04