steam-run: add derivation

[abbradar]

Things done:

• Tested using sandboxing (nix-build --option build-use-chroot true or nix.useChroot on NixOS)
• Built on platform(s)
  • NixOS
  • OS X
  • Linux
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

More

This adds a new steam-run script, which gives one ability to run something with our assembled Steam Runtime in environment. This should come very handy for all kinds of games downloaded from various places (GOG, for example). Steam Runtime is the de facto expected Linux userspace for commercial games, so this should make running them much more easy.

Example:

> steam-run ./run-game.sh

This also makes hopefully small but incompatible change to userFHSEnv, renaming importMeta argument to meta (a more standard name). userFHSEnv is mostly an internal function used by buildFHSUserEnv, so this should not be a problem.

Merging this in few days unless there are any remarks.

[mention-bot]

By analyzing the blame information on this pull request, we identified @hrdinka, @svanderburg and @edolstra to be potential reviewers

[jagajaga]

Can you give an example to test?

[abbradar]

I cannot give you a game itself ~_^ -- but if you have anything downloaded from GOG, Humble Bundle and others who distribute stand-alone Linux games, running steam-run ./start-the-game (where ./start-the-game depends on the actual game) should make it Just Work.

[jagajaga]

Ok, got it!