[PhpProcess] Environment variables must be passed through to the child PHP process

When a PhpProcess is instantiated from [BrowserKit][Client], the environment variables fail to reach the child process, causing the test to fail on HHVM.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | none
| License       | MIT
| Doc PR        | none