DAOS-19340 test: set FI log level to warn for rebuild tests (#19046)

[kccain]

For the daos_test rebuild tests, set engine environment variables needed to capture warnings in logfiles for connection disruptions (e.g., when engines stopped, restarted, and reintegrated). The goal is especially to log when there are unusual connectivity disruptions (e.g., after an engine restart, specific engine endpoint pairs unable to communicate, often involving a common engine's single CaRT context).

Test-tag: daos_core_test_rebuild
Skip-unit-tests: true
Skip-fault-injection-test: true
Skip-test-rpms: true

Steps for the author:

• Commit message follows the guidelines.
• Appropriate Features or Test-tag pragmas were used.
• Appropriate Functional Test Stages were run.
• At least two positive code reviews including at least one code owner from each category referenced in the PR.
• Testing is complete. If necessary, forced-landing label added and a reason added in a comment.

After all prior steps are complete:

• Gatekeeper requested (daos-gatekeeper added as a reviewer).

[github-actions]

Ticket title is 'daos_test/rebuild.py:DaosCoreTestRebuild.test_rebuild_36 - timeout waiting for rebuild'
Status is 'Reopened'
Labels: '2.8.0rc2,ci_2.8_weekly,daily_test,verbs_provider'
https://daosio.atlassian.net/browse/DAOS-19340

[kccain]

@daos-stack/daos-gatekeeper I've added the request_for_2.6.6 label to the Jira ticket for this one. Not sure what constraints there are on landing anything to release/2.6. this is a test configuration only change that could be helpful in diagnosing future occurrences of rarely reproducing connectivity problems with engine stop/start/reintegrate testing and verbs provider configurations in CI. The problem has not been seen in src/tests/ftest/daos_test/rebuild.py testing on release/2.6 branch (yet) - it has been seen on 2.8 and master.