feat(nested_env): make `Env::insert()` generic over value

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Bencher Report

Branch	524/merge
Testbed	github-actions

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Benchmark	perf:task-clock	Benchmark Result msec (Result Δ%)	Lower Boundary msec (Limit %)	Upper Boundary msec (Limit %)
laze -C RIOT build --global --generate-only	📈 view plot 🚷 view threshold	9,537.18 (+2.67%)	9,022.04 (94.60%)	9,556.51 (99.80%)

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