nix-portable-009

Update nix, proot & support aarch64-linux

• update nix to 2.5.1
• update proot t 5.3.0
• support aarch64-linux
• remove variable NP_MINIMAL
• new variable NP_GIT to specify path to git executable
• always install the git from nixpkgs on first use, except when NP_GIT is defined.