Test-Grid: Should be a flake

The VM that runs the Test-Grid runs (https://github.com/tahoe-lafs/infrastructure/tree/main/testgrid.tahoe-lafs.org symlinked into /etc/nixos .  It updates to the latest NixOS "channel" configured as the root user's channel (which is out of the repository configuration).

To go with the times, the test-grid configuration should be a pure/hermetic flake.