Use --force when running "git checkout main"

The latest nightly builds have been failing with the error:

Error: Error running git checkout main:
error: The following untracked working tree files would be overwritten by checkout:
        .nobackup
Please move or remove them before you switch branches.
Aborting

This is because a recent commit added a .nobackup file at the root of tor-browser-build (tor-browser-build#40996 (closed)), but we already had that file manually created on tb-build-06.

To avoid this kind of error, we can run git checkout main with the --force option (in TBBTestSuite/TestSuite/TorBrowserBuild.pm).