ci: Add nix install action

[NickLarsenNZ]

This fixes the last PR #323 which errors with:

TASK [Operator [airflow-operator] regenerate crate2nix] ************************
fatal: [localhost]: FAILED! => {"changed": true, "cmd": ["make", "regenerate-nix"], "delta": "0:00:00.204998", "end": "2024-02-21 12:43:50.841070", "msg": "non-zero return code", "rc": 2, "start": "2024-02-21 12:43:50.636072", "stderr": "bash: line 1: nix: command not found\nmake: *** [Makefile:136: regenerate-nix] Error 127", "stderr_lines": ["bash: line 1: nix: command not found", "make: *** [Makefile:136: regenerate-nix] Error 127"], "stdout": "nix run -f . regenerateNixLockfiles", "stdout_lines": ["nix run -f . regenerateNixLockfiles"]}