fix: Linux CI test env propagation

[sjmiller609]

Summary

• forward CI into the sudo-backed Linux go test invocations in make test
• do the same for the manual Linux guest-memory test entrypoint for consistency
• restore the repo's intended CI timeout behavior for Linux integration tests

Root cause

Linux CI runs tests through make test, which uses sudo env ... go test ....
That command forwarded PATH, DOCKER_CONFIG, and the prewarm vars, but not CI.
As a result, integration-test helpers like integrationTestTimeout(...) behaved as if they were running locally, so sub-45s waits stayed at 20s instead of widening on CI.

This surfaced in the latest main merge as a flaky failure in TestStandbyAndRestore during the initial create-to-running wait.

Verification

• verified on deft-kernel-dev that the child sudo test process now receives CI=true
• ran make test TEST=TestStandbyAndRestore TEST_TIMEOUT=90m remotely with CI=true
• also ran targeted remote repro loops before the fix to confirm the failure was contention-sensitive rather than a deterministic restore bug

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Note

Low Risk
Low risk Makefile-only change that only affects how test processes inherit environment variables under sudo. Main risk is unintended behavior differences in local Linux runs if CI is set in the caller environment.

Overview
Fixes Linux test runs invoked via sudo env ... go test to preserve the CI environment variable, aligning integration-test behavior (e.g., CI-specific timeouts) with actual CI execution.

Applies the same CI forwarding to the test-guestmemory-linux manual integration test target for consistency.

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