Only wait when the `VK_WAIT_FOR_DEBUGGER` env var is set.
Long term I want to move away from relying on environment variables, but this is currently consistent with the `VK_DEBUGGER_PORT` env var which also enables the debugger.

Bug: b/148135662
Change-Id: I990d907d7570569e9d6716cf5d13e9846f4a5c02
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/40429Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>