CI: Empty environment parameters are super-flaky on Appveyor.

[ryan-roemer]

Appveyor keeps hitting these failures:

  1) bin/builder-core builder run runs with empty base + non-empty archetype config value:
     AssertionError: expected 'ECHO - undefined - EMPTY\n' to include 'string - EMPTY'
      at C:\projects\builder\test\server\spec\bin\builder-core.spec.js:972:27
      at C:\projects\builder\test\server\spec\bin\builder-core.spec.js:65:5
      at C:\projects\builder\test\server\spec\bin\builder-core.spec.js:52:9
      at C:\projects\builder\node_modules\async\lib\async.js:52:16
      at C:\projects\builder\node_modules\async\lib\async.js:361:13
      at C:\projects\builder\node_modules\async\lib\async.js:52:16
      at done (C:\projects\builder\node_modules\async\lib\async.js:246:17)
      at C:\projects\builder\node_modules\async\lib\async.js:44:16
      at C:\projects\builder\node_modules\async\lib\async.js:358:17
      at C:\projects\builder\test\server\spec\bin\builder-core.spec.js:47:7
      at FSReqWrap.readFileAfterClose [as oncomplete] (fs.js:380:3)

because it's really touchy about a non-existent environment variable vs. an empty string. The test has a config of:

      base.mockFs({
        ".builderrc": "---\narchetypes:\n  - mock-archetype",
        "package.json": JSON.stringify({
          "config": {
            "_test_message": "" // base with empty strings wins.
          }
        }, null, 2),
        "node_modules": {
          "mock-archetype": {
            "package.json": JSON.stringify({
              "config": {
                "_test_message": "from archetype"
              },
              "scripts": {
                "echo": ECHO + " >> stdout.log"
              }
            }, null, 2)
          }
        }
      });

Status: Disabling test for just windows until a more permanent fix is encountered.