Execute GitHub action on pull requests too and fix coverage

[kinow]

This is a small change with no associated Issue.

Noticed that after marking a PR as ready for review it was not showing that the GItHub action had run successfully. Then found a similar issue (that repository has a lot of useful things for actions), and turns out I also did not include the on: [pull_request], only on: [push].

EDIT: added an extra commit to fix the coverage reports. Will have to add a secret token to the repo settings too, as documented in the Codecov github action docs: https://github.com/codecov/codecov-action

One review should to 👍

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Does not need tests (why? build only).
• No change log entry required (why? e.g. invisible to users).
• No documentation update required.

[kinow]

Best way to review is just read the one-line change, and confirm that in this PR the GitHub action appeared as a "check" (same way that it used to appear for Travis/Codacy/etc).

Should be visible somewhere 👇