test(kkernel): restore daemon-spawn test environment

[ohdearquant]

Summary

• centralize every process-global environment variable mutated by daemon-spawn tests
• capture the complete environment set and working directory in one scope guard
• restore every value on normal return or panic and cover the full mutation set with a regression test

Verification

• cargo fmt --all -- --check
• cargo clippy -p kkernel --all-targets -- -D warnings
• cargo test -p kkernel

Regression test: daemon_spawn_env_guard_restores_every_mutated_variable

Fixes #2046