Remove anemone fork pin once Speyll/anemone#41 merges

[stackptr]

#3 pins the `anemone` theme input to PinkNoize's zola-0.23/Tera-v2 migration branch instead of upstream `Speyll/anemone`, because the needed fix hasn't merged yet: Speyll/anemone#41.

Once #41 merges upstream:

• Point the `anemone` input in `flake.nix` back to `github:Speyll/anemone` (drop the fork/rev pin)
• Run `nix flake update anemone`
• Verify `nix build` / `nix flake check` still pass