ci: pass placeholder token to cache-nix-action

cache-nix-action@v7 reads its token input as required in both restore
and save; its github.token default is empty under ngit-ci (no
GITHUB_TOKEN is injected), so both phases threw "Input required and not
supplied: token" and, hidden by continue-on-error, the Nix store cache
never restored or saved. Any non-empty value satisfies the check and is
never transmitted while purge (default off) stays disabled.

Validation: matches the fix applied to ngit-ci's own workflows and its
documented snippet; the next push-triggered run should show the
restore/save steps succeeding instead of failing fast.

.ngit/act/workflows/rust_ci.yaml

@@ -28,6 +28,9 @@ jobs:
         with:
           primary-key: nix-${{ runner.os }}-${{ hashFiles('**/*.nix', 'flake.lock') }}
           restore-prefixes-first-match: nix-${{ runner.os }}-
+          # cache-nix-action requires a token even with purge disabled;
+          # github.token is empty under ngit-ci, so any non-empty value works.
+          token: unused
       - name: Restore Rust build cache
         continue-on-error: true
         uses: actions/cache@v4