refactor(tvix/nix-compat): remove use of lazy_static

This is now supported in the standard library via std::sync::LazyLock, but
requires some manual shuffling around of code.

I found at least one dead variable along the way, which I deleted.

Change-Id: I8600c87c49078fb5ff72671994c77b919259e67b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12608
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Autosubmit: tazjin <tazjin@tvl.su>

tvix/nix-compat/Cargo.toml

@@ -41,7 +41,6 @@ optional = true
 criterion = { workspace = true, features = ["html_reports"] }
 futures = { workspace = true }
 hex-literal = { workspace = true }
-lazy_static = { workspace = true }
 mimalloc = { workspace = true }
 pretty_assertions = { workspace = true }
 rstest = { workspace = true }