Remove unnecessary unsafe blocks from environment variable operations in tests

[coderabbitai]

Context

This issue was identified during the review of PR #82 and requested by @leynos.

Description

The test function run_respects_env_override_for_ninja in tests/runner_tests.rs uses unsafe blocks when calling std::env::set_var and std::env::remove_var. These functions are safe to call and do not require unsafe blocks.

The unnecessary unsafe blocks should be removed while maintaining the existing logic that:

• Saves the original environment variable value
• Sets the override for testing
• Runs the test
• Restores the original state

References

• PR: Refactor CLI build handling into helper functions #82
• Review comment: Refactor CLI build handling into helper functions #82 (comment)
• Issue: Remove unnecessary unsafe blocks from environment variable operations in tests #87

Files affected

• tests/runner_tests.rs (Lines 166-202)

[leynos]

The functions std::env::set_var and std::env::remove_var are unsafe in Rust 2024 and must be marked as such.