The gitea-actions-runner's host-scheme job processes use the
service's environment PATH, not the NixOS login-shell PATH.
Without this, 'nix flake check' and similar steps fail with
'nix: command not found'.
- add pkgs.nix to environment.systemPackages (ensures the binary
is in /run/current-system/sw/bin)
- set explicit PATH on gitea-runner-hive service covering both
/run/current-system/sw/bin and /nix/var/nix/profiles/default/bin
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