[Nix-dev] help with Travis builds

[Azul]

n00b here

Trying to sort out PR: https://github.com/NixOS/nixpkgs/pull/15986

while "it works on my machine", travis barks out at me with:
https://travis-ci.org/NixOS/nixpkgs/builds/135414974

so I looked into the travis builds to find out what they were doing, and
come up with this to replicate them locally.

This Vagrantfile fails with the same error as Travis

```
vbox_version = `VBoxManage --version`

Vagrant.configure(2) do |config|
  config.vm.box = "ubuntu/trusty64"
  config.vm.network "private_network", ip: "192.168.33.96", nic_type:
"virtio"

  config.vm.provider "virtualbox" do |vb|
     vb.memory = "3072"
     vb.customize ["modifyvm", :id, "--natdnshostresolver1", "on"]
     vb.customize ["modifyvm", :id, "--natdnsproxy1", "on"]
     vb.customize [ "guestproperty", "set", :id,
"/VirtualBox/GuestAdd/VBoxService/--timesync-set-threshold", 10000 ]
     vb.customize ["modifyvm", :id, "--nictype1", "virtio"]
     vb.customize ["modifyvm", :id, "--paravirtprovider", "kvm"] if
vbox_version.to_f >= 5.0
     vb.customize ["storagectl", :id, "--name", "SATAController",
"--hostiocache", "on"]
     vb.customize ["modifyvm", :id, "--ioapic", "on"]
     vb.customize ["modifyvm", :id, "--cpus",
`#{RbConfig::CONFIG['host_os'] =~ /darwin/ ? 'sysctl -n hw.ncpu' :
'nproc'}`.chomp]

     vb.linked_clone = true if Vagrant::VERSION =~ /^1.8/
  end
  config.vm.provision "shell", inline: <<-SHELL
    groupadd nixbld
    usermod -G nixbld vagrant
    apt-get update
    apt-get install -y build-essential git
    test -e /swapfile || ( dd if=/dev/zero of=/swapfile bs=1M count=2048 &&
 mkswap /swapfile )
    swapon /swapfile
  SHELL

  config.vm.provision "shell", privileged: false, inline: <<-SHELL
     git clone https://github.com/NixOS/nixpkgs.git
     cd nixpkgs
     export TRAVIS_PULL_REQUEST=15986
    ./maintainers/scripts/travis-nox-review-pr.sh nix
    ./maintainers/scripts/travis-nox-review-pr.sh nox
    ./maintainers/scripts/travis-nox-review-pr.sh build
  SHELL
end
```


But this Vagrantfile builds my PR correctly

```
vbox_version = `VBoxManage --version`

Vagrant.configure(2) do |config|
  config.vm.box = "ubuntu/trusty64"
  config.vm.network "private_network", ip: "192.168.33.96", nic_type:
"virtio"

  config.vm.provider "virtualbox" do |vb|
     vb.memory = "3072"
     vb.customize ["modifyvm", :id, "--natdnshostresolver1", "on"]
     vb.customize ["modifyvm", :id, "--natdnsproxy1", "on"]
     vb.customize [ "guestproperty", "set", :id,
"/VirtualBox/GuestAdd/VBoxService/--timesync-set-threshold", 10000 ]
     vb.customize ["modifyvm", :id, "--nictype1", "virtio"]
     vb.customize ["modifyvm", :id, "--paravirtprovider", "kvm"] if
vbox_version.to_f >= 5.0
     vb.customize ["storagectl", :id, "--name", "SATAController",
"--hostiocache", "on"]
     vb.customize ["modifyvm", :id, "--ioapic", "on"]
     vb.customize ["modifyvm", :id, "--cpus",
`#{RbConfig::CONFIG['host_os'] =~ /darwin/ ? 'sysctl -n hw.ncpu' :
'nproc'}`.chomp]

     vb.linked_clone = true if Vagrant::VERSION =~ /^1.8/
  end
  config.vm.provision "shell", inline: <<-SHELL
    groupadd nixbld
    usermod -G nixbld vagrant
    apt-get update
    apt-get install -y build-essential git
    test -e /swapfile || ( dd if=/dev/zero of=/swapfile bs=1M count=2048 &&
 mkswap /swapfile )
    swapon /swapfile || echo
  SHELL

  config.vm.provision "shell", privileged: false, inline: <<-SHELL
     git clone https://github.com/NixOS/nixpkgs.git
     cd nixpkgs
     git config --global user.email "me at example.com"
     git config --global user.name "Your Name"

     git remote add Azulinho https://github.com/Azulinho/nixpkgs.git
     git fetch Azulinho
     git merge Azulinho/update_libgit2_to_v24
     export NIXPKGS=$PWD
     export TRAVIS_PULL_REQUEST=15986
    ./maintainers/scripts/travis-nox-review-pr.sh nix
    ./maintainers/scripts/travis-nox-review-pr.sh nox
    . $HOME/.nix-profile/etc/profile.d/nix.sh
    nix-build nixos/release.nix -A options --show-trace
    nix-build pkgs/top-level/release.nix -A tarball --show-trace
    nix-build $PWD -A python27Packages.pygit2
    nix-build $PWD -A python35Packages.pygit2
  SHELL
end
```

Before I dig into reading about nox and getting into the grittier details,
does anyone has a hint for what could possibly be happening here?
happy to pay back with a merged PR

- azul
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