fix: runtime `NSS_DB_PATH` and `Once`-guarded `fixture_init` #164

Job	Run time
Determine toolchains	8s
cargo clippy (macos-15, nightly)	5m 33s
cargo clippy (ubuntu-24.04, stable)	4m 16s
cargo clippy (macos-15, stable)	5m 34s
cargo clippy (windows-2025, 1.90.0)	5m 32s
cargo clippy (macos-15, 1.90.0)	5m 33s
cargo clippy (ubuntu-24.04, 1.90.0)	5m 4s
cargo clippy (ubuntu-24.04, nightly)	4m 49s
cargo clippy (windows-2025, stable)	5m 34s
cargo clippy (windows-2025, nightly)	6m 9s
	48m 12s

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