add instructions for NixOS and improve them for Android

[joee]

Summarize CA installation instructions for the current versions of NixOS and Android.

[HenkVanMaanen]

Thanks, that helped me getting it working on NixOS!

[Nikola-Milovic]

Does this work for you guys?

error: 1 dependencies of derivation '/nix/store/ziqx73jpirlgb6kbpg0z27gzpbvj87nx-etc.drv' failed to build
error: 1 dependencies of derivation '/nix/store/nvrqfl6vpjjdxw3xjxv7b9bjx1890hnb-nixos-system-workstation-25.05.20251128.9a7b80b.drv' failed to build
┏━ 1 Errors: 
 ⋮ 
┃        >   File "/nix/store/3p8d2ad8igdb546c598jqh3q5lqxnhai-python3.12-buildcatrust-0.3.0/bin/.buildcatrust-wrapped", line 9, in <module>
┃        >     sys.exit(main())
┃        >              ^^^^^^
┃        >   File "/nix/store/3p8d2ad8igdb546c598jqh3q5lqxnhai-python3.12-buildcatrust-0.3.0/lib/python3.12/site-packages/buildcatrust/cli.py", line 243, in main
┃        >     sys.exit(cli_main(sys.argv[1:]) or 0)
┃        >              ^^^^^^^^^^^^^^^^^^^^^^
┃        >   File "/nix/store/3p8d2ad8igdb546c598jqh3q5lqxnhai-python3.12-buildcatrust-0.3.0/lib/python3.12/site-packages/buildcatrust/cli.py", line 182, in cli_main
┃        >     raise FileNotFoundError(f"Bundle not found: {bundle_path}")
┃        > FileNotFoundError: Bundle not found: /home/nikola/.local/share/mkcert/rootCA.pem

The file is there

> ls ~/.local/share/mkcert/
rootCA-key.pem  rootCA.pem

Configured as such

    security.pki.certificateFiles = [ "/home/${config.${namespace}.user.name}/.local/share/mkcert/rootCA.pem" ];