Merge branch 'master' into core-updates

Change 'nix-upstream' submodule URL.

* .gitmodules: Use https, not http.  Reported by
  Pjotr Prins <pjotr.public12@thebird.nl>.

Merge branch 'nix-integration'

Conflicts:
	tests/guix-package.sh

build: Add Nix as a sub-module.

* .gitmodules, nix-upstream: New files.