Allow exec 'env' to inherit from 'process.env'

[scothis]

When executing a command Node uses process.env by default. If the 'env' option is specified, Node assumes that the complete environment is provided.

As Scripted exec configurations are declarative, and it's practically impossible to correctly configure an environment declaratively, Scripted should mix the declared env vars into the 'process.env'.

For example, this is my .scripted config:

"exec": {
  "onKeys": {
    "cmd+b": {
      "cmd": "npm test",
      "cwd": "${projectDir}",
      "name": "Test Project",
      "env": {
        "BUSTER_TEST_OPT": "--color none --reporter specification"
      }
    }
  }
}

Before this patch, Node is unable to spawn the child process because the environment is incomplete. After this patch, it's able to run the command with 'process.env' as the environment, augmented with 'BUSTER_TEST_OPT'.

[aclement]

Kris - can you look at merging this.