lazy_static has better ergonomics at the call/access sites (it returns a
reference to the type directly, whereas with once_cell we get a static Lazy<T>
that we must dereference instead) but the once_cell api is slated for
integration into the standard library [0] and has been the "preferred" way to
declare static global variables w/ deferred initialization. It's also less
opaque and easier to comprehend how it works, I guess?
(Both `once_cell` and `lazy_static` are already in our dependency tree, so this
should have no detrimental effect on build times. It actually negligibly
*improves* build times by not using macros, reducing the amount of expansion the
compiler has to do by a miniscule amount.)
[0]: https://github.com/rust-lang/rust/issues/74465
The nix crate had all its default features enabled, which included features that
are not present under BSD. We should only enable the select subset of crate
features that we know are available cross-platform (or else use conditional
targeting in Cargo.toml to only enable Linux-only features when compiling for
Linux targets).
For now, it seems we can just use the nix crate with all features disabled as it
still builds under Linux and FreeBSD in this state.
