Don't assume the location of PHPUnit coverage reports

[kasparsd]

In https://github.com/xwp/wp-dev-lib/blob/420f1bac828e105a19a904cab97d042891d4b84e/scripts/check-diff.sh#L804-L816 we pass $(coverage_clover) which is equal to --coverage-clover build/logs/clover.xml if can_generate_coverage_clover returns 1 which is the case whenever a Coveralls config file is found in the project root and coverage task is enabled.