🔒 Fix Arbitrary Command Execution via OIL_SYSTEM_PREFIX in testing

[undivisible]

🎯 What: The resolve_install_dest_without_prefix test spawned the test executable but only removed the OIL_SYSTEM_PREFIX environment variable, allowing it to inherit all other variables.
⚠️ Risk: Inheriting environment variables in self-executing tests could lead to unexpected behavior or arbitrary command execution via variables like LD_PRELOAD or other shell configurations.
🛡️ Solution: Modified the test to use .env_clear() instead of .env_remove("OIL_SYSTEM_PREFIX") to ensure the subprocess starts with a clean environment.

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PR created automatically by Jules for task 17662094030822357210 started by @undivisible

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Note

Low Risk
Test-only change to subprocess environment setup; no production install-path or runtime behavior is modified.

Overview
Hardens the resolve_install_dest_without_prefix test when it re-executes the test binary in a subprocess (when OIL_SYSTEM_PREFIX is set in the parent). The subprocess launcher now uses .env_clear() instead of .env_remove("OIL_SYSTEM_PREFIX"), so the child no longer inherits the full parent environment.

That closes a test-only footgun where variables like LD_PRELOAD or other inherited config could alter subprocess behavior or widen arbitrary execution risk during self-spawned test runs.

^{Reviewed by Cursor Bugbot for commit c0a42c4. Configure here.}

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