Nix flake enablement for gardenlogin

[vasu1124]

What this PR does / why we need it:

We want to enable gardenlogin for the nix community.

# with Nix (macOS, Linux, and Windows)

# development version
nix profile install github:gardener/gardenlogin

gardenlogin --help
kubectl gardenlogin --help

[petersutter]

Thanks for the contribution.
I've added some feedback, but I'm not familiar with Nix. Generally, I'm wondering what problem we're trying to solve with yet another installer (to put it simply). Or are we just adding to the maintenance burden (hardcoded vendor hash that has to be updated on every dependency update, go version, nixos version)?

[SuperSandro2000]

splitVersion, major, minor can be repalced with lib.versions.major/minor.

As you can see in the syntax highlighting that no longer works, this is not really how nix does things but I think it would work. The idomatic nix way would be something like:

            gitCommit = if (self ? rev) then
              self.rev 
            else
              self.dirtyRev;

[SuperSandro2000]

I'm wondering what problem we're trying to solve with yet another installer (to put it simply).

Nix also allows things like nix run github:vasu1124/gardenlogin which most other package managers can only dream off.
Also now other flakes or nixos configuration can just reference this repo to install gardenlogin and don't need to manually install it or use tools like brew which lack reproducibility and a generall way of defining things declaratively which makes building devops tooling in a central place not easier.

[vasu1124]

Generally, I'm wondering what problem we're trying to solve with yet another installer (to put it simply).

The nix ecosystem allows for a simplified developer onboarding and enhanced team productivity. There are already teams that use nix for this purpose and have created nix functions/packages for all our gardentools. Let's be frontrunners and not laggards.

Or are we just adding to the maintenance burden (hardcoded vendor hash that has to be updated on every dependency update, go version, nixos version)?

Let's tackle that issue in a next step.