ci: run nix copy

Author: jfroche

.github/workflows/nix-build.yml

@@ -51,7 +51,7 @@ jobs:
           #!/usr/bin/env bash
           set -eouf
           export IFS=' '
-          echo /nix/var/nix/profiles/default/bin/nix copy --to 's3://nix-postgres-artifacts?secret-key=nix-secret-key' \$OUT_PATHS
+          /nix/var/nix/profiles/default/bin/nix copy --to 's3://nix-postgres-artifacts?secret-key=nix-secret-key' $OUT_PATHS
           EOF
           sudo chmod +x /etc/nix/upload-to-cache.sh
       - name: Install nix