test(cli): isolate install plan receipt fixture

[aaiyer]

What does this PR do?

Fixes the cli_install_plan_receipt_no_mutation_issue388 test harness so it cannot inherit a developer or CI runner CODEX_HOME while using a temporary HOME fixture.

The observed 4,976-byte LeakSanitizer report was a harness-cleanup symptom, not a production allocation defect:

• With inherited CODEX_HOME, production correctly resolved the Codex path outside the temporary fixture.
• The receipt assertion then failed before free(json).
• The same pre-fix sanitizer binary passed all 222 CLI tests when run with CODEX_HOME unset.

The fixture now saves, unsets, and restores CODEX_HOME. It also records the receipt and no-mutation results, then frees the JSON, restores the environment, and removes the temporary directory before reporting an assertion failure.

Reproduce-first evidence

• RED: with inherited CODEX_HOME, ASan/UBSan CLI execution failed at the install-plan receipt assertion and LeakSanitizer reported 4,976 bytes allocated through yyjson in cbm_build_install_plan_json.
• CONTROL: the same pre-fix binary with CODEX_HOME unset passed 222/222 CLI tests without a leak.
• GREEN: the patched focused CLI ASan/UBSan suite passes 222/222 under the inherited CODEX_HOME environment, without a sanitizer report.

Checks

• One signed-off commit; scripts/check-dco.sh upstream/main..HEAD passes
• Focused CLI ASan/UBSan suite: 222/222
• scripts/lint.sh --ci
• make -f Makefile.cbm security
• scripts/check-no-test-skips.sh
• clang-format 20.1.8 dry-run
• git diff --check
• Independent focused review: 0 High / 0 Medium / 0 Low
• Full local scripts/test.sh: started but stopped after the relevant CLI phase while unrelated deep/matrix suites continued pathologically long; it did not reach a final summary. GitHub CI is authoritative for the exhaustive suite.

Scope

Only tests/test_cli.c changes. Production behavior is unchanged.