CI builds: Checkout main branch in a robust way

[benjaoming]

Adds an -f switch to force resetting the branch in cases where Circle CI had already fetched it.

I'm still not sure about the exact mechanisms of the built-in Circle CI checkout step. At first I introduced this in a PR where main wasn't found as a reference - and it worked, the branch was added and pre-commit could compare changes in the PR to main.

But then in a later PR after this change, I got errors when main already exists.

git branch --track main origin/main

fatal: a branch named 'main' already exists

https://app.circleci.com/pipelines/github/readthedocs/readthedocs.org/6345/workflows/892de4a3-b7de-4f65-80c2-d7f4643d81f3/jobs/15361

I think the -f switch should do the work:

       -f, --force
           Reset <branchname> to <startpoint>, even if <branchname> exists already. Without -f, git branch refuses to change an existing
           branch. In combination with -d (or --delete), allow deleting the branch irrespective of its merged status. In combination with
           -m (or --move), allow renaming the branch even if the new branch name already exists, the same applies for -c (or --copy).

[benjaoming]

I believe this can be merged given that a reasonable follow-up is discussed here: #9795