fix: runtime `NSS_DB_PATH` and `Once`-guarded `fixture_init` #165

Job	Run time
Determine toolchains	7s
cargo clippy (ubuntu-24.04, 1.90.0)	4m 33s
cargo clippy (windows-2025, stable)	7m 33s
cargo clippy (ubuntu-24.04, nightly)	4m 56s
cargo clippy (windows-2025, nightly)	9m 49s
cargo clippy (macos-15, nightly)	5m 45s
cargo clippy (macos-15, 1.90.0)	5m 12s
cargo clippy (windows-2025, 1.90.0)	9m 27s
cargo clippy (macos-15, stable)	6m 13s
cargo clippy (ubuntu-24.04, stable)	4m 32s
	58m 7s

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