build: set PYTHON in nix flake so that the docs build

[notjustanyusername]

I notice the nix flake check was removed in #8743. Without the check in CI the flake will keep breaking.

• The changelog has been updated in the relevant commit(s) according to the guidelines.
• Tests have been added or modified to reflect the changes.
• Documentation has been reviewed and updated as needed.
• Related issues have been listed and linked, including any that this PR closes.
• Important All PRs must consider how to reverse any persistent changes for tools/lightning-downgrade

[Andezion]

Do you have an actual build log or reproduction showing the docs/msggen/grpc-proto step failing on current master without this change? Since the flake check isn't in CI anymore, that's currently unverifiable from CI history - it'd help to see the concrete failure this is meant to resolve, or confirm whether this is speculative hardening rather than an observed fix

[daywalker90]

I think this fix is more appropriate #9423. What do you think?