fix: runtime `NSS_DB_PATH` and `Once`-guarded `fixture_init` #166

Job	Run time
Determine toolchains	4s
cargo clippy (macos-15, 1.90.0)	6m 37s
cargo clippy (macos-15, nightly)	4m 46s
cargo clippy (macos-15, stable)	6m 11s
cargo clippy (ubuntu-24.04, 1.90.0)	4m 48s
cargo clippy (windows-2025, nightly)	9m 43s
cargo clippy (windows-2025, stable)	8m 0s
cargo clippy (ubuntu-24.04, stable)	4m 1s
cargo clippy (ubuntu-24.04, nightly)	5m 29s
cargo clippy (windows-2025, 1.90.0)	10m 30s
	1h 0m 9s

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