travis: Add dummy environment variables as build labels

Travis doesn't have explicit support for labeling builds
in a build matrix, so use a dummy environment variable as a poor man's
label.

Before: https://travis-ci.org/NixOS/nixpkgs/builds/264390928
After: https://travis-ci.org/NixOS/nixpkgs/builds/264499784

.travis.yml

···
           script:
               - ./maintainers/scripts/travis-nox-review-pr.sh nixpkgs-verify nixpkgs-manual nixpkgs-tarball nixpkgs-unstable
               - ./maintainers/scripts/travis-nox-review-pr.sh nixos-options nixos-manual
+          env:
+            - BUILD_TYPE="Test Nixpkgs evaluation & NixOS manual build"
         - os: linux
           sudo: required
           dist: trusty
           before_script:
               - sudo mount -o remount,exec,size=2G,mode=755 /run/user
           script: ./maintainers/scripts/travis-nox-review-pr.sh nox pr
+          env:
+            - BUILD_TYPE="Build affected packages (Linux)"
         - os: osx
           osx_image: xcode7.3
           script: ./maintainers/scripts/travis-nox-review-pr.sh nox pr
+          env:
+            - BUILD_TYPE="Build affected packages (macOS)"
 env:
     global:
         - GITHUB_TOKEN=5edaaf1017f691ed34e7f80878f8f5fbd071603f