ci: use treefmt2 with flag --ci

Update treefmt to version 2.

This adds the following flags for CI usage:
"--no-cache, --fail-on-change and adjusting some other settings best suited to a CI".
See: https://treefmt.com/usage

.forgejo/workflows/check.yml

@@ -10,7 +10,7 @@ jobs:
 
       - name: Check formatting
         run: |
-          nix --accept-flake-config --access-tokens '' develop --command treefmt --fail-on-change
+          nix --accept-flake-config --access-tokens '' develop --command treefmt --ci
 
       - name: Run flake checks
         run: |

flake.nix

@@ -107,7 +107,7 @@
               nvfetcher
               shellcheck
               shfmt
-              treefmt
+              inputs.unstable.legacyPackages.${system}.treefmt2
               nixos-generators
               inputs.unstable.legacyPackages.${system}.opentofu
               terraform-backend-git