Report and Log nightly version used in `test-plan-generator` agent for debugging purposes.

[jpangas]

We need to report the version being used in

bugbug/agents/test-plan-generator/hackbot_agents/test_plan_generator/firefox_install.py

Line 1 in beb81a2

"""Download and install a prebuilt Firefox Nightly for the agent to drive."""

May need to be done for the other agents as well that use firefox nightly.