🐛 Ensure that if a fetch fails it retries with authentication in the same way as clone

[joshjennings98]

Description

Problem:

• cd-assert-news creates a temporary clone of the repo
• that clone later does a git fetch
• the fetch used the normal remote URL without authentication
• in a private repo, that failed with GitHub credential errors

Fix:

• it tries a normal fetch first
• if that fails, it retries using the authenticated GitHub URL built from GIT_TOKEN

This is the same way htat clone() works.

Test Coverage

• This change is covered by existing or additional automated tests.
• Manual testing has been performed (and evidence provided) as automated testing was not feasible.
• Additional tests are not required for this change (e.g. documentation update).