ci: Disable coverage tests in PRs that do not touch code

[cason]

Namely,

1. codecov/project (Codecov Report, which in particular requires the author to be "an activated member of this organization on Codecov".
2. Coverage / Integration (pull_request), which also checks Rust code

This request is particular relevant for spec and documentation PRs, where we don't touch any Rust code.

[cason]

Note that PRs can still be merged when those tests fail or are incomplete, so this is not really blocking anyone.