Add missing toml package to post-submit CI

[vanbasten23]

Currently, the post-submit CI fails with error:

+ mkdir lcov
+ cp .coverage lcov/
+ coverage-lcov --data_file_path lcov/.coverage
Traceback (most recent call last):
  File "/opt/conda/bin/coverage-lcov", line 5, in <module>
    from coverage_lcov.cli import main
  File "/opt/conda/lib/python3.8/site-packages/coverage_lcov/__init__.py", line 3, in <module>
    import toml
ModuleNotFoundError: No module named 'toml'
Error: Process completed with exit code 1.

This PR adds the missing package.

[vanbasten23]

Thanks for the review!