Be more atomic in our operations with example repos

[myitcv]

At the moment we follow this sort of pattern at the start of an example:

now=$(date +'%Y%m%d%H%M%S_%N')
githubcli repo renameIfExists $GITHUB_ORG/$r ${r}_$now
githubcli repo transfer $GITHUB_ORG/${r}_$now $GITHUB_ORG_ARCHIVE
githubcli repo create $GITHUB_ORG/$r

But this means that for the entire of the regen of the example we are "missing" the real repo.

Instead we should switch to doing renames and transfers at the end of the example which will minimise downtime.

[myitcv]

Actually unclear whether we can get this to work, because doing work into a temporary repo will mean that resolution of import paths will fail.