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.