Add support for Nix/NixOS

[lopsided98]

This is part of my effort to add support for Nix to ROS. It depends on ros-infrastructure/rospkg#171.

The Nix package manager is often used with NixOS, but it runs on any Linux distribution as well as macOS. In order to fit cleanly into the ROS infrastructure, I am pretending that Nix and NixOS have the traditional package manager and distribution relationship. This means that the rosdep OS key for Nix is nixos, even though this key may be used on multiple distributions. When packages are built, the build sandbox prevents OS detection from working anyway, so the ROS_OS_OVERRIDE environment variable is used to force the correct value.

rosdep will not be particularly useful to end users, because its imperative package management style does not fit Nix very well. Instead, it will be used to resolve dependencies when generating packages using superflore.

I am working on adding support for Nix to various ROS projects:

• https://github.com/lopsided98/rospkg/tree/nixos-support
• https://github.com/lopsided98/rosdep/tree/nixos-support
• https://github.com/lopsided98/superflore/tree/nixos-support

I maintain a Superflore generated nixpkgs overlay and have successfully built over 3600 ROS packages on a Hydra build server (not yet publicly accessible)

I also made a small cleanup to the Arch Linux platform.

[tfoote]

lgtm, it won't pass CI until the rospkg update is merged and released first.

[dirk-thomas]

it won't pass CI until the rospkg update is merged and released first.

rospkg 1.1.10 has been released with the referenced change.

[codecov-io]

Codecov Report

Merging #697 into master will increase coverage by 0.05%.
The diff coverage is 88%.

@@            Coverage Diff             @@
##           master     #697      +/-   ##
==========================================
+ Coverage   75.47%   75.52%   +0.05%     
==========================================
  Files          32       33       +1     
  Lines        2968     2987      +19     
==========================================
+ Hits         2240     2256      +16     
- Misses        728      731       +3

Impacted Files	Coverage Δ
src/rosdep2/platforms/arch.py	90% <100%> (ø)	⬆️
src/rosdep2/__init__.py	78.57% <100%> (+0.38%)	⬆️
src/rosdep2/platforms/nix.py	83.33% <83.33%> (ø)

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[nuclearsandwich]

In the future when adding support for new platforms we need to remember to bump the rospkg version requirement to the appropriate version.