Superproject commitbot fixes

Note: Before merging, please add a gha secret "CI_PAT" .

In a personal github account, create a token that has permission to push to this superproject.
At https://github.com/boostorg/boost/settings/secrets/actions create a gha secret "CI_PAT", and enter the token.

From https://github.com/orgs/community/discussions/25702 "If an action pushes code using the repository’s GITHUB_TOKEN, a new workflow will not run"

Updates in this PR:

Modify the git token to be secrets.CI_PAT
Only run on a cron schedule, not from pushes
Fix the git submodule url variable. During testing it had been set to a full url such as https://github.com/boostorg/math.git. In production the format is ../math.git. Convert to a url.

cc. @glenfe @pdimov

boostorg / boost