Need choice of NixOS as platform

I often run into issues where the somewhat leaky hermeticity of bazel hits hard hermeticity enforcements in NixOS, so things that sorta compile with bazel on 'regular' Linux machines are breaking on NixOS.

This often results in BCR modules being broken on NixOS because they exploit some hidden assumptions of the target system. Problem of course is, that this platform is not available as presubmit platform in BCR -- so many modules are broken silently with an uphill battle to fix them one PR at a time by users of a hermetic system.

It would be a real benefit if it was possible to have a NixOS CI platform to choose from (in particular in the [BCR](https://github.com/bazelbuild/bazel-central-registry) so that it is possible to fix things and be sure that they don't backslide.

I suspect the BCR choices of available presubmit platforms hinges on `bazelbuild/continuous-integration`, hence I file an issue here.

I see that there was work towards a Nix set-up available (#1703) , but then looks like it got removed again. I don't know all the context, but it would be good if this can be re-activated (and stay stuck). I am happy to help if needed.

This is what I see happened before:
* https://github.com/bazelbuild/continuous-integration/pull/1779 Container with Nix was added
* https://github.com/bazelbuild/continuous-integration/pull/1799 changed to use Ubuntu + nix package manager.
* https://github.com/bazelbuild/continuous-integration/pull/1827 Looks like Ubuntu+Nix got removed again :(

CC @rickvanprim as he did a lot of work here

Start by reviewing the Nix-related history in continuous-integration PRs 1779, 1799, and 1827, along with the BCR presubmit-platform configuration referenced in the issue. Determine how a NixOS-capable CI platform could be restored and remain available; done means BCR can select it for presubmit validation.

Written by the indexing model from the issue text.