cli/image: make history test timezone-independent

[keeltrace]

AI generated PR; managed by a human. Feel free to close without merging or make any and all requests and I will update the PR - keeltrace handler

What I changed

Make TestNewHistoryCommandSuccess set time.Local directly to UTC for the duration of the test and restore the previous location with t.Cleanup.

Why

The test previously used t.Setenv("TZ", "UTC"), but another test in the same package calls .Local() first. Once Go initializes time.Local, changing TZ no longer changes the already-cached location. On non-UTC hosts, the package test could therefore compare a local RFC3339 timestamp against the UTC golden file and fail.

Verification

• Before the change, TZ=America/Los_Angeles ./scripts/with-go-mod.sh go test ./cli/command/image -count=1 fails in TestNewHistoryCommandSuccess/non-human.
• After the change, the package passes under UTC, America/Los_Angeles, and Asia/Tokyo.
• TZ=America/Los_Angeles ./scripts/with-go-mod.sh go test -race ./cli/command/image -count=1 passes.
• ./scripts/with-go-mod.sh go test ./cli/... -count=1 passes.
• ./scripts/with-go-mod.sh go test ./... -count=1 passes all normal packages; E2E packages require TEST_DOCKER_HOST and are unavailable on this host.
• make -f docker.Makefile test-unit is unavailable on this host because Docker Buildx is not installed.

[thaJeztah]

Perhaps it's worth keeping the comment (but can be moved to the new code).

It's unfortunate indeed that TZ is only read once for a process (with a sync.Once) so there's no clean way to override it for a test 🤔