ci: stabilize TMPDIR for Go test cache

[davdhacs]

Description

Set TMPDIR=/tmp in the unit-tests workflow env block.

Go's test cache (computeTestInputsID) hashes every environment variable that the test binary reads via os.Getenv(). These reads are recorded in the test binary's internal log (via testing/internal/testdeps) and replayed by computeTestInputsID on subsequent runs to verify that inputs haven't changed.

Approximately half of our test packages call os.TempDir() (directly or through imported libraries), which reads TMPDIR. On GHA, each job gets a unique TMPDIR — so testInputsID differs between the master push (cache save) and PR run (cache restore), causing Phase 2 cache misses for every affected package.

Config	Cache hits
Master baseline (no fix)	571/762 (75%)
With TMPDIR=/tmp (from master cache)	693/762 (91%)
With TMPDIR=/tmp (own cache, stable)	712/762 (93%)

GODEBUG=gocachehash=1 on CI (PR #21276) captured 128,527 testInputs hash lines across 762 packages. 817 of those reference TMPDIR.

User-facing documentation

• CHANGELOG.md is updated OR update is not needed
• documentation PR is created and is linked above OR is not needed

Testing and quality

• the change is production ready: the change is GA, or otherwise the functionality is gated by a feature flag
• CI results are inspected

Automated testing

No test changes. This fixes the CI test caching infrastructure.

How I validated my change

• GODEBUG=gocachehash=1 on CI captured hash inputs confirming TMPDIR in testInputsID (PR debug(ci): testInputsID hash capture for cache investigation #21276, run 27789436790)
• 10-branch parallel A/B experiment (PRs test(ci): cache-test-a — baseline: v2 trim + coverprofile #21199-test(ci): cache-test-j — coverprofile, NO modfix (control) #21215) isolated variables
• PR ci: stabilize TMPDIR for Go test cache #21277 (experiment version) cherry-picked 11 master commits demonstrating 93% cache hits across real code changes vs 75% baseline
• Local reproduction confirmed TMPDIR is not hashed when stable

[openshift-ci]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[coderabbitai]

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✨ Finishing Touches

🧪 Generate unit tests (beta)

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• Commit unit tests in branch davdhacs/tmpdir-fix

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[github-actions]

🚀 Build Images Ready

Images are ready for commit 3f08c9a. To use with deploy scripts:

export MAIN_IMAGE_TAG=4.12.x-239-g3f08c9a93c

[openshift-ci]

@davdhacs: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/gke-qa-e2e-tests	3f08c9a	link	false	/test gke-qa-e2e-tests

Full PR test history. Your PR dashboard.

Details

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[tommartensen]

What is a "Phase 2 cache" in this context?