Make coveralls CI action success not required for CI

[LMNTL]

Checklist

1. If you've added code that should be tested, add tests
2. If you've changed APIs, update (or create!) the documentation
3. Ensure the tests pass
4. Make sure that your code lints and that you've followed our coding style
5. Write a title and, if necessary, a description of your work suitable for publishing in our release notes
6. Mention any related issues in this repository (as #ISSUE) and in other repositories (as kobotoolbox/other#ISSUE)
7. Open an issue in the docs if there are UI/UX changes

Description

Resolves an issue that caused python CI to fail.

Notes

This PR is used to fix an issue where the Run coveralls for back-end code step would consistently fail on some branches, causing CI to fail.

Internal thread cross-reference

Unlike #4717, this PR temporarily resolves the issue by adding continue-on-error to the step, since a failure generating the coverage report is unrelated to whether the code passes tests, builds, lints, etc. The coverage % badge will still be updated, since the 422 issue appears to be sporadic and hasn't affected beta or main yet - the main difference will be that some PRs don't submit a coverage report.

[LMNTL]

Note: CI is failing due to an unrelated CI error with translations. See successful run with latest code here.