Automate the process of updating the "tahoe-lafs-master" source

[exarkun]

With #281 we added CI test coverage for ZKAPAuthorizer vs a supposedly-recent version of Tahoe-LAFS master. For this to actually be a recent version, it needs to be updated sometimes. At the moment, the update process is manual. It would be less ongoing work if some automation did this for us. A nice workflow might be for some system to generate a PR against the main branch that bumps the revision to whatever is current at the time the PR is created and for this to happen regularly (eg, once a week). It might make sense to delay this work until ZKAPAuthorizer finds its long-term home (which may not be on GitHub) - to avoid building a lot of GitLab-specific automation that becomes obsolete.

[exarkun]

Now we integrate against Tahoe-LAFS master using a Nix Flake input which is even easier to update. Just run nix flake lock --update-input tahoe-lafs-dev.