The environment variables were being modified incorrectly, introducing
blank lines, which Windows doesn't like, leading to the spawned deqp
process having a blank env var table.

Change-Id: Id034594219397809667670f9f7fb26f67ee7d87c
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/40888Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>