Introduce installSecretFn to shorten script when many secrets are used.

[catwith1hat]

Fixes https://github.com/ryantm/agenix/issues/227. Tested with 100 secrets.

Please let me know, if you have some feedback on the style. I didn't try too hard, because I don't know what you prefer.

[tomeon]

@catwith1hat -- thanks for tackling this. I too am hitting the "Argument list too long" error. Wanted to draw your attention to NixOS/nixpkgs#274765, where I am attempting to solve this problem for any and all activation scripts.

[catwith1hat]

I addressed your comments. Please take another look.

[catwith1hat]

Sorry, did I break the Darwin integration? It's not obvious from the logs on the CI what is happening there. Initially, I didn't pay much attention to the broken tests as many github projects have occassionally broken CIs. But now I see that my other PR https://github.com/ryantm/agenix/pull/242 does succeed with Darwin, so I am now doubting my hypothesis. Sadly, I have no Darwin machine to test or debug.