tests/ci: Collect environment information

For reproducing errors or test results it is important to gather
environment information. These environments are divided into two parts.
One part is in which environment the artifacts were built into and the
second part is in which environment the tests were run.

This patch collects thesee information and saves into a .env file in
the metadir. After this patch lands we will also need to change our
jenkins job where we are uploading the artifact to a central location.
This environment file should also be stored along with the artifact.
For easier location the image basename and the environment basename are
kept same.

Approved by: lwhsu
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D54247