Partially revert "build(deps): bump cachix/install-nix-action from 26 to 27"

This partially reverts commit ab7becf047.

The pkgs/by-name check doesn't work for newer Nix versions yet,
see https://github.com/NixOS/nixpkgs-check-by-name/issues/78

.github/workflows/check-by-name.yml

@@ -94,7 +94,7 @@ jobs:
           base=$(mktemp -d)
           git worktree add "$base" "$(git rev-parse HEAD^1)"
           echo "base=$base" >> "$GITHUB_ENV"
-      - uses: cachix/install-nix-action@ba0dd844c9180cbf77aa72a116d6fbc515d0e87b # v27
+      - uses: cachix/install-nix-action@8887e596b4ee1134dae06b98d573bd674693f47c # v26
         if: env.mergedSha
       - name: Fetching the pinned tool
         if: env.mergedSha