Fix red develop 1597a850bb16

The merge lane found `develop` red at `1597a850bb16dd8e720806071dfd9eb9ebec536e` on its own: no queued PR merges until it is fixed. Failing (tests and gate stages): - `repo/tools/tests/test_deb_upgrade.py::test_fresh_install` - `repo/tools/tests/test_deb_upgrade.py::test_a_remove_deletes_the_venv_dpkg_never_shipped` - `repo/tools/tests/test_deb_upgrade.py::test_a_first_install_that_cannot_fetch_wheels_fails_configure_with_the_sentence` - `repo/tools/tests/test_deb_upgrade.py::test_a_rebuild_that_cannot_fetch_wheels_keeps_the_previous_venv` - `repo/tools/tests/test_deb_upgrade.py::test_an_upgrade_from_the_build_before_stops_no_live_work` - `repo/tools/tests/test_deb_upgrade.py::test_the_updater_converges_to_the_desired_version` - `repo/tools/tests/test_deb_upgrade.py::test_an_update_re_creates_a_deleted_runner_user` - `repo/tools/tests/test_deb_upgrade.py::test_an_update_removes_an_emptied_session_user_and_restores_the_uid_boundary` - `repo/tools/tests/test_deb_upgrade.py::test_an_update_keeps_a_set_session_user` - `deb-upgrade-container` The whole gate log: artifact `11890` on lane task 11130. Make `develop` pass its full gate again. Open a PR as usual: the merge lane merges a PR that fixes some of these and adds no failure even while `develop` is red.

Fixed red develop: deb-upgrade stager now substitutes ${JAAH_PYTHON_FLOOR} from platform pins (added by a5a1baf98). Container suite 9/9 passed, quick gate green. PR #7281 queued to the merge lane. Memory: none applied.