Support for NixOS

### Description of the feature request:

At the moment, `rules_gitops` will not work when used on NixOS host.
This feature request, asks for adding built-in support for aforementioned operating system.

__Details__:
Due to its nature, NixOS rigidly separates package dependencies, down to ELF interpreters - everything
is symlinked and kept isolated. The system itself, does not and should not have files like `/lib/ld-linux.so.2`, nor has it libraries present in default, global namespaces.

Consequently, binaries which are produced in 'default manner' often fail to work on NixOS (quite frequent, due to no `RPATH` or default ELF interpreter).
More details can be [read here](http://sandervanderburg.blogspot.com/2018/10/auto-patching-prebuilt-binary-software.html).

This incompatibility could be a source of major pain, while using most of Bazel rules, however it is possible to
configure golang sdk, so that it will built binaries correctly - see the details [here](https://github.com/tweag/rules_nixpkgs#nixpkgs_go_configure).

Unfortunately, pre-configuring golang sdk __is not enough__ to make the `rules_gitops` compatible, as it downloads pre-built
`kustomize` binary. This differentiates it from `bazelbuild/rules_k8s` or `bazelbuild/rules_docker` where such pre-configuration is enough.

This feature request, postulates to detect if rules are executed on NixOS, and - if and only if - they are indeed executed on NixOS, use built into the OS mechanism for obtaining kustomize binary.

### Feature requests: what underlying problem are you trying to solve with this feature?

Currently I have to apply custom patches to `rules_gitops` to make it work on NixOS machines - I would rather treat it in the same manner other, similar Bazel tools (rules_k8s, rules_docker).

It may seem trivial to add few patches in one project, but keeping track of it in many WORKSPACES is tedious.

### Bugs: what's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

1. Use NixOS (20.09) host
2. Clone the example project: [link](https://github.com/AleksanderGondek/bazel-k8s-resources-mngmt)
3. Remove patches fixing the issue [WORKSPACE lines 33 and 34](https://github.com/AleksanderGondek/bazel-k8s-resources-mngmt/blob/master/WORKSPACE#L33)
4. Run ```$ bazel build //kubeview:kustomize-dev```

### What operating system are you running Bazel on?

NixOS 20.09

### What's the output of `bazel info release`?

```release 3.3.1- (@non-git)```

### If `bazel info release` returns "development version" or "(@non-git)", tell us how you built Bazel.

It was installed by nix package manager [link](https://search.nixos.org/packages?channel=20.09&show=bazel&from=0&size=50&sort=relevance&query=bazel).

### Snippet of the `WORKSPACE` file that includes rules_gitops rules.

```
...

load("@bazel_tools//tools/build_defs/repo:git.bzl", "git_repository")
load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")

git_repository(
name="com_adobe_rules_gitops",
commit="354e7d3341f05e076f286663731f18caf1e62340",
remote="https://github.com/adobe/rules_gitops.git",
shallow_since="1611050644 +0000",
)
load("@com_adobe_rules_gitops//gitops:deps.bzl", "rules_gitops_dependencies")
rules_gitops_dependencies()

...
load("@com_adobe_rules_gitops//gitops:repositories.bzl", "rules_gitops_repositories")
rules_gitops_repositories()
```

### Have you found anything relevant by searching the web?

* Why most binaries need patching on NixOS - [link](http://sandervanderburg.blogspot.com/2018/10/auto-patching-prebuilt-binary-software.html)
* Running golang-based Bazel rules on NixOS - [link](https://github.com/tweag/rules_nixpkgs#nixpkgs_go_configure)

### Any other information, logs, or outputs that you want to share?

In connection with closing of [issue 52](https://github.com/adobe/rules_gitops/issues/52), this would make `adobe/rules_gitops` support NixOS nearly out-of-the-box.

Comienza con gitops/deps.bzl y gitops/repositories.bzl, y luego reproduce el fallo usando el ejemplo de NixOS 20.09 y la configuración de WORKSPACE proporcionados. Rastrea cómo se obtiene el binario precompilado de kustomize y considera la ruta específica de NixOS solicitada. Se considera terminado cuando el ejemplo se compila en NixOS sin parches personalizados de WORKSPACE, mientras el comportamiento existente permanece sin cambios en el resto.

Escrito por el modelo de indexación a partir del texto del issue.